Fantasy
Wish fulfillment

As can be seen, fantasy as basic pleasure or escape and fantasy as wish-fulfillment or the satisfaction of desire have strong ties, but there is a subtle difference in the shades of meaning between the two. The role that fantasy plays in the satisfaction of desire is subverted and belittled by the use of the term wish-fulfillment, but the term is so pervasive that I would be remiss if I did not mention that this satisfaction of desire is what most people are criticizing when they talk about the wish-fulfillment aspect of fantasy.
There is an implication in the terms satisfaction and fulfillment, however, that these critics fail to see: namely, the existence of a lack or need that must be fulfilled or satisfied. "For fantasy characteristically attempts to compensate for a lack resulting from cultural constraints: it is a literature of desire, which seeks that which is experienced as absence and loss" (Jackson 3). In addition to those desires arising from cultural constraints that fantasy fulfills, Tolkien posits in "On Fairy Stories" that fantasy also fulfills certain deeper, more basic desires: "among these [operations of fantasy] are the satisfaction of certain primordial human desires. One of these is to survey the depths of space and time. Another is (as will be seen) to hold communion with other living things" (Tree 103). Now, if these desires are primordial, as Tolkien argues, then they are by extension intrinsic and unalterable. So fantasy, in providing the outlet for and expression of these desires serves a valuable purpose.
Does this justify its study and explication? From a psychological perspective, almost certainly. From a literary perspective, maybe, but before such a judgment can be made, we need to go further and examine the final purpose of fantasy: to reveal truth.