Role-playing, in my opinion, fulfills the creative desires of our minds. It is akin to being a child again, when anything is possible.
Ahh, straight from the Cassian himself, I couldn't have put it better.
Role-playing is about limitless potential. You can do anything, be anything or anyone. You want to be a general of the fleet? Sure. You want to be a mad scientist, inventing new poisons or tampering with alien DNA? Absolutely. Doctor? Yup. Cute bunny girl? Why not. Secret Agent? Of cours-
Well okay, maybe not /as/ cool as the real thing. But close enough. I think you get my point.
Role-playing is something we've probably all done to some degree and it is a phenomenal tool for creative exercising and social interaction. Sometimes it can even be a challenge. It's like a choose your own path story, where other people and random events spark the life the pages you will base your next move on. It is everchanging and it can frankly, be really exciting because of that.
For some people, it is about adding a bit more meaning to their gaming experience. Taking your avatar one step further then the shifting band of pixels in a colourful world adds another depth of meaning to the game you are playing. These often include those who do not consider themselves hard core roleplayers, or maybe not even roleplayers at all.
Take for instance, Star Wars the Old Republic. Through interactive storytelling many people cannot help but develop an affinity for their character. Why do they do what they do? Why do they choose to say this, and not that? What motivates them? I know from my own personal experience that some of my friends who do not consider themselves roleplayers actively got excited about their characters' motivations for their interactions in their environment and with each other. We joked and laughed, nothing terribly serious, but from it three unique personalities were born. Suddenly the stories we were playing through were not just the story that everyone was playing. In fact, it was no longer even just unique to ourselves. It was unique to our own interactive drama between the three of us that brought something more to the table.
Ultimately the rewards, benefits and reasoning for role-playing lie within the eye of the beholder. Perhaps it is an unintential personification of your avatar. Perhaps it is a little bit of you poking through that digital screen, eager to explore new and unique territory. Perhaps it is the yearning to be something amazing and different.