To answer this question, we need to look beyond the individual at humanity as a group or society of individuals. I believe personality has evolved as a problem-solving mechanism. Our unique personalities determine that we all think and hence solve problems in unique and different ways. In an evolutionary sense, this diverse method of solving problems is highly effective. If we had only one method of problem solving there would be a large, if not infinite, number of solutions that would be outside of our problem solving capabilities.
Emotion is, to a large degree, an emergent system.
Its use must be looked at in terms of its interaction with society rather than in isolation to gain a better understanding of its reason for being. We can look at a corporation as an example. Here, at the top of the hierarchy, we have a CEO who is bold and fearless, making broad decisions with little regard to details. At the other end of the hierarchy, we might find someone who is fearful of the unknown, is timid, and has great respect for details. The organization, and in a greater sense society, needs both types of people and the many others in between to function efficiently.