My overly honest answer:
This is something I've thought about a few times. A question I've asked myself on more than one occasion when I log off and think, why do I bother with this? It really doesn't do me much good, and some days it can quite literally consume my day away, when I have such little available time anyways. Though, it's always been pretty tough to really nail the exact reasons.
Largely, there is a part of this that is sentimental to me. I practically grew up in forums and chat rooms like these. I've made countless friends, and experienced loss of those friends as they've moved on, or I may have. I can recall stories and characters that are now just mere memories as their pages have been eradicated from the net in more than one occasion, and yet I still write hoping to recreate some of those, that I can never really get back. But, to hopefully create new memories as well.
The community is something I typical adore as well. Even in parts of some forums, namely the Arena sections, where things can often get very intense and cumbersome, there is something about the bonds that form that keeps me coming back for more. Which is really weird in some cases, because some of those same things is where some people could look and want to run away. On every forum I've ever been on, the Arena has been my absolute favorite place, because as much argument that goes on in that one small section, there is a brotherly connection that forms between you and the others that you simply can't quite reach in some other areas of RP forums. These guys and the occasional gal are my brothers and sisters, and I wouldn't have it any other way.
Yes, there is the escapism aspect. My life is filled with ups and downs and straight out roller coasters, and even when it can get that way roleplaying, there is this ground that is present, that I can hold onto whenever things get just a little too shaky. Furthermore, I have never had much of a sense of self. Roleplaying has allowed me to explore that. Specially when in life, there are so many things that can hold you back from just straight out being yourself, I can come on here and really just be whoever I want to be, and no one really cares or judges me on that, unless I'm just being a complete jerk and then they have no issues calling me out on it. Haha. For real though, I can come and lose myself in writing and not really have a care in the world about what is going on right beside me.
And of course I do love writing, though my favorite part is character creation. I can spend all day on characters if I had the time. Characters are so diverse and complex and I can control how I want them to be and how they develop and grow. Stories can be the same way, but I think it goes back to that sense of person. In exploring these idea characters in my mind, I develop a further understanding of my own sense of self. Heck, some of my characters are more elaborate than what you can tell from reading a character sheet, because, how can you really define every aspect of a person without also having the aspect of trial and tribulation? Anyways, I'm just rambling there.
And sometimes it's just a sense of relief. The ability just to sit here like right now and just type whatever comes to my mind, instead of keeping everything locked away, constantly trying to keep those around me happy while I'm completely and utterly miserable. Sometimes it helps just to write. It doesn't even matter what it is about. It's almost like a form of therapy (as much as I hate to apply that word to it.). It's sometimes just what I need to do, to be able to get myself through the day. Just Write.