I'm surprised no one has mentioned what I believe to be perhaps the most important benefit of playing D&D, at least in terms of a relative scarcity of it in other activities: use and development of imagination. So many of our entertainments and leisure activities--not to mention work--are simulative, involving watching or interacting in a pre-made visual field (e.g. film, tv, video games); or they are sense-based, involving engaging in a physical-sensory experience (sports, outdoors, sex, etc); or they are social-relational (going out, parties, etc). If they are mental, they are usually analytic-intellectual, involving problem-solving, analysis, and so on. Very few activities, at least in this day and age, involve direct and focused use of the imagination....especially with the overall decline in reading.
So, yeah: imagination. It is also what separates tabletop RPGs from related activities that don't engage the imagination as much, if at all, like board and video games.