Why do we need emotions?
The idea that people could get along without emotion is the "big lie." Emotions are our system of dealing with the unknown, our system of deciding to push harder, run away, or go for it. We have lots of them. They can be intensely contradictory but this all creates a means of processing data that has yet to be recreated by engineers.
Basically, our emotions operate like a Prediction market. The bidders are our memories. And, the call to bid is queued by similarity of perceptions/situations to events or perceptions stored in our memory. The process of recalling is very fast because it essentially bypasses our cognitive thought process though the same mechanism that habits use.
However, emotions are both the source and product of habituation. Emotions provide both the rewards and punishments that trigger, modify and suppress habits. Thus as a perception is being recorded into memory, along with its perceptual components (auditory, visual, kinesthetic, olfactory/gustatory) the emotional loading is recorded too.
When this memory is "triggered" by an event or perception that is similar to the event or perception stored in memory, it is recalled. The closer the match, the stronger the "signal" relative to the initial recollection.
This is extremely similar to but not the same as the concept of the "indexed" query. However, in terms of computing, the processes the brain uses to compress and access data are very similar to concepts being used today in databases. Much of our sleeping brain activity is given over to the process of compressing data, indexing it, and associating it with existing data via associating it with already existing objects and classes and only saving the "interesting" variations.
Slightly off topic but neuroscience is slowly approaching this same conclusion from the biological side.
Best detailed hypothesis I've seen of the function of sleep! by Frank Heile on Consciousness Science
https://consciousness_science.quora.com/Best-detailed-hypothesis-Ive-seen-of-the-function-of-sleep
Emotions are different than Appetites/Urges. Both of these classes of sensation give rise to interior perceptions of pleasure or pain.
Appetites and Urges are the expression of the body's demand for sustenance, liquid, air, warmth/coolness(temperature moderation), skin integrity (itch/pain), urination, evacuation, and procreation. These are instinctual level responses that can override all rational thought in acute distress though in varying degrees. You can will yourself to starve to death, but you cannot hold your breath until you die. (These sensations can be grouped into distressed or sated with gorged/bloated/dizzy/crowded as possibility for the appetites - food, liquid, air and comfort.)(over-hydration distress is rare though with normal kidney function)
Emotions are a class of sensations apart from these basic urges of life. They can be grouped into three broad classes based on degree of spontaneity. But, this classification is more of a gradient than from impulsive to disciplined responses with emotional responses being in the center of this continuum. These groups are Impulse, Emotion and Attitude. This continuum is related to the amount of damping or constraint that restrains the amplitude and speed of reaching that amplitude of the onset of an emotional response. More later.
In a broad generality emotions can exist in one of three exclusive classes, Anxiety, Aggression or Attraction. This is the good old "fight or flight" but a third state "seek or approach" is also a possibility. John C. Loehlin identified this third state in attempting to develop simple model of behavior that was used to generate a computer model called Aldous. And, this is very much the case for human emotion. In fact, as humans we actually live in this state up to 80% of our waking days. Aggressions and Anxieties are relatively rare but this is just "relatively" as 20% is still hours of each day.
Beyond this, Hunter B. Shirley, isolated four separate domains of motivational action that form the basis of human emotional reasoning. These domains, Affinity, Identity, Mobility and Diversity represent basic motivational goals of the organism. This system allows each potential action, opportunity or threat to be analyzed from each of these perspectives simultaneously (in parallel) and the result of this analysis is yielded as fast as our systems can pattern match the perceptual cue to the recollected experience or experience class. This recollection process is very much almost automatic and the result is that fast.
This is a fact of human psychology that marketing experts are more than happy to use to get you to buy their products.
The conscious mind can take 5 times as long to catch up to the "gut" feeling you already have, and very often, your "reasoning" is just attempting to justify that feeling, not to reign it in. Which is also why, folks can often be heard saying some really silly things, as they try to justify their strong feelings but cannot command the facts because the "reasoning" part was completely skipped.
This is not to say that this response is not reasoning though. In fact, the emotional result is the consequence of the perception in question being pattern matched against all recollections from four competitive goal sets simultaneously. This is a wonder and an amazing facility.
I am going to focus on the center ground of this set of feelings after just touching on the extremes. Impulses are the result of matches where few alternate or offsetting experiences are recalled and thus children's behavior is dominated by impulsive displays and actions. As experience builds, countering and offsetting models and experiences temper this initial "rush" to judgement so that the result is more considered, the intensity of the feeling is dampened and focused, this sort response is called emotion here. As, even more experience builds, one can develop highly disciplined responses or even jaded responses which are called attitude.
In the emotional range of expression, each of the four goal sets or domains of reasoning can have one of three states, Aggression, Anxiety or Attraction. This results in 12 basic emotions that all other emotions we recognize are combinations, underdamped (impulses), critically damped (emotions) or overdamped (attitudes). Impulsive Aggression or Anxiety often is at very high magnitude, lower levels resolve into emotion typically but impulsive emotions that are tinged with anxiety are often called "slimy" and when tinged with aggression, "crude."
Here is a chart to help you recognize each of these twelve basic emotions
This is a quick map of the twelve basic emotions and how these compound into the great global compound emotions of Love, Hate, and Grief.
These basic emotions can form compounds of two or four emotions depending on motivational correlation, and these compound can be of equal or differing intensities.
So just like a color wheel, this system spins out definitions of emotions and meaning as we consider mixed emotions in this model. A partially ambivalent emotion is one that combines and attraction with an aggression or anxiety. These are handy ways of softening otherwise outright or naked displays of anxiety or aggression. But, the human mind interprets this very well and the "catch" is telegraphed.
So instead of Admiration = Pride + Affection you get things like Condescension = Affection + Contempt. And, thus like a decoder ring, this chart also allows you to deconstruct that display of Exasperation into Anger + Affection. And, then you know, they are frustrated about something, and you can focus on that by converting Anger into Desire.. and thus Exasperation into Hope.
Beyond the partial ambivalent emotions are a set of fully mixed emotions or ambivalent emotions that mix aggression and anxiety in a couple. I will do global compounds and global ambivalences some other time.
Fully ambivalent emotions are gut wrenching difficult experiences for the person experiencing them and for people coping with the contradictory messages they send. So rather than Sincerity = Pride + Curiosity, Hypocrisy = Shame + Disdain which is a false kind of sincerity. Snobbery is also another form of false sincerity. And, envy is a false form of determination.
However, when you look at these as cues to motivation and also as the preparation for action, emotions literally are our means of interpreting our own control signals. They are a feedback system that tells the mind just what it is thinking and doing. They also provide reward and punishment for building habits and achieving goals. In short, without them, we would be lost.