I thought there was some specific science to this but. I guess I'll just give my 2 cents.
When I was younger, role play and I were never really on the same foot. I went from being absolutely enthralled, to despising it, to now... being okay with it. I mostly write stories and poems, and I used to scoff at role play. Why? I guess I never understood its importance to people, and what it does. When I was in band, I used to scoff at choir people until I learned that partaking in choir will help to perfect my skills. I fell in love with choir in my senior year. That situation is kind of why I role play now... to hone my skills. Yet, as I go along, I find that I am enjoying myself.
I will be honest and say I used to be one of those role play snobs. I used to be extremely picky with grammar and sentence structure, and overall the people involved. I used to be one of those snobs until I matured, and I grew mentally. Iwaku, and past places I have been to within this last year, have taught me that if you want to fix something... then by damn, help yourself by helping the person causing the problem. There is nothing more awful than sitting and stewing over ill feelings. Within this past year, I have seen what it has done not only to myself, but the people around me who have practiced it. It ruins things. And in turn, I am learning to be more aware of what I say, instead of relying on how I say it through tone. To me, it just seems like a cycle of positivity. If someone is having problem with their grammar, then you give them some pointers or direct them to someone who can help. I in exchange, get a chance to practice being helpful without appearing condescending. The person who was having problems, then gets information and a chance to refine their writings.
The appeal to me, in a role play forum, is helping people. It's not just about the role plays, the characters, the plots [although some are delightfully entertaining] it is that by helping, I am making someone's life a bit easier.
The fact that roleplaying in general is a facet of my life, and that through it, I not only become a better writer, but a better person is what makes role playing so worthwhile. It is not just the roleplays that make it so, but the community. You cannot role play without a partner, and why have a single partner when you can have a myriad of teachers helping one wade through the mucks and trials of every day life, in character and out?
Roleplay is a therapy. I write often when the mood is darkest, because to me, that is the only way I can channel through everything when I feel emotionally cornered. It helps to get all those thoughts and feelings out on paper in a palpable form. They not only become words written by me, through me, byt they become a scenery, a character, a plot, an idea. They -are- me.
Despite role play being a therapy, and a life changing experience, I find that it also is a challenge. You aren't writing by yourself, you're writing with a bunch of people. They may post something you never expected them to do, and so you have to find a way to counter it, react or otherwise find a way to keep the role play flowing. It challenges the mind, and it certainly keeps those juices flowing.
The options are endless when it comes to role play, it's just exploring all those options that make it interesting.