Current Affect as Signal for Future Expected Emotion
What is the purpose of emotion, and how does emotion function in human psychology? A widely held view is that emotions exist to cause behavior (e.g., Cosmides & Tooby, 2000; Frank, 1988; Frijda, 1986; Izard & Ackerman, 2000). Many theories of emotion use the standard example of fear. An early human who lacked fear might stand still, curiously watching a tiger approach, so that he would be eaten. Such non-fearful individuals would thereby take themselves out of the gene pool. As a result, today's population would only be descended from forebears who experienced fear. The fear made them run away from the tiger and other dangers, with the beneficial result that they survived longer and were better able to reproduce. It's a very persuasive just-so story.
Although the idea that emotions are for guiding behavior is intuitively appealing, in fact, the evidence for it is remarkably weak and frequently contradictory. Research on emotion has shown plenty of effects on thought processes, judgment, and decision making—but effects on actual behavior are few and far between. Two influential reviews of the literature by Schwarz and Clore (1996, 2007) remarked on the contrast between the extensive evidence about cognitive effects of emotion and the minimal evidence of behavioral effects. In the second review, the authors concluded that "The immediate effects of emotion ... are more mental than behavioral" (2007, p. 39).
The idea that the main purpose of emotion is to cause behavior was aggressively rejected by Baumeister, Vohs, DeWall, and Zhang (2007) on both conceptual and empirical grounds. They reiterated some critiques of the view that emotion is for causing behavior. These include the idea that there are not enough emotions for all the possible behaviors, so that emotions cannot contain specific directions for how to act. (As Schwarz & Clore  pointed out, fear might well cause someone to do a variety of things other than start running: The person might hide, listen to the weather report, sell stocks, work harder at a job, purchase insurance, or telephone home.) They include the fact that emotions arise slowly, indeed much too slowly to guide behavior in a fast-moving situation. (For example, if after first seeing a cat, a mouse waited until its body generated a state of fear complete with physiological arousal, and then for its brain to perceive that state and understand it as fear, and then send the signal to run, it wouldn't get around to budging fast enough to avoid becoming the cat's lunch.)
Another objection to the theory that the purpose of emotion is to cause behavior is contained in the popular view, well confirmed by empirical studies, that actions driven by current emotional state often end up being irrational and even self-destructive, which means that causing behavior could not be the main purpose of such emotions. Natural selection cannot favor self-defeating actions, insofar as self-destruction is inimical to survival and reproduction. To the extent that emotions push people into self-defeating actions, motivating action is not the evolved purpose of emotion. This might be offset by large amounts of beneficial behaviors motivated by emotion; but it's hard to find much evidence of that, other than the imaginary example of fear making one run away and survive.
Instead, Baumeister et al. (2007) proposed that the purpose of emotion is to stimulate reflection. Emotion directs attention to important events and stimulates the mental processing of them. A computer moves on to the next task once the current one is finished, but humans dwell on things that are over and done with. By continuing to ruminate about them, including veridical replays and counterfactual replays, humans may learn useful lessons that can guide behavior helpfully in future situations.
Moreover, people learn from these experiences how they will feel if they act in particular ways in particular situations. Gradually they become able to anticipate what the emotional outcome will be. People generally operate so as to promote positive emotions and avoid or minimize bad ones, and so once they can effectively forecast how they will feel, they can adjust their behavior effectively.