I recently stumbled upon this talk on TED and found it fascinating.
I once read Chris Crawford's The Art of Computer Game Design in which he alluded to the importance of play as grounds for dealing with the trials of adult life. In particular he focused on the animal kingdom's use of play as a means of training cubs for combat. The theory was sound in that through the use of play Lion cubs might learn a greater awareness of their environment, develop the confidence to explore and ultimately develop the skills to take down prey.
Stuart Brown in his talk here suggests that play is much more than that.
A fascinating talk.