Basically emotions help us be honest with ourselves about how our current perceptions are filtered by our history of perceptions. Emotions are an entirely internal event, they know nothing of the world outside you and everything about your perception of the world. If you think you see a monster, your emotions will reflect monster appropriate feelings even if there is no monster. For emotions, there is no reality, just perception.
For example, when I feel angry, it is never because I am in a situation that requires or justifies anger. And this is true even if anger is highly appropriate to the situation. My anger is never a response to the situation, it is instead a response to my perception of the situation. My perception is also significantly colored by my history with similar perceptions. Therefore when I am angry, the proper first response should not be directed at the circumstances but rather at my perception.
Sometimes I have found myself getting angry when I think I am hearing someone say something abusive, but upon paying closer attention I discover I have misunderstood what they were saying. When my perception authentically changes, then so does my emotional state. My anger was not a response to what they were saying but rather a reflection to what I perceived them to be communicating.
"Is this circumstance truly what I think it is? Am I being prompted by past conclusions or is this an opportunity to see this situation differently from how I have looked at it before. I am angry so obviously I am seeing this circumstance as one that is threatening or unjust, but am I being fair, or am seeing only what I've grown accustomed to paying attention to in what looks to me to be this sort of situation?"
Only after applying reality checks to my history and to my current perceptions can I decide what the most appropriate response might be. Being honest about what my perceptions really are is easier when I allow my feelings to be the first clue. Emotions are a reflection of learned (or mislearned) meaning that we have come to ascribe to perceptual moments over time. Without feelings we can live in denial of what we really think things mean. To grow toward more useful meaning I must start that journey from where I am. I can fully trust my emotions to keep me honest about where I am now in that journey to more useful meaning.
The person who learns to delay reaction to feeling in order to reassess perception is a person far more free to feel whatever they honestly feel. They can be intensely happy and keep a poker face in a card game and beyond angry while keeping their cool. Adding this additional step to the dynamic of "emotions motivating actions" can add room for wisdom.
Because emotions are based on perception and because I can only trust my perceptions to be potentially useful, I can therefore never trust that what I feel is based on reality. Nonetheless, I can fully trust my emotions to help me be honest about what my perceptions are doing with reality. And that eventually helps me develop better perceptions with which to make sense of the world and helps me experience emotions far easier to live with.