I RP as an exercise in creative writing and self-improvement.
I explore characters I'm interested in exploring largely because they represent something about myself or about the world that I want to spend some time critically examining. People who've RPed with me in the past can probably attest that my characters are all very, very different; they're not self-inserts, so this might not seem obvious. But they each either have a different slice of me, or a slice of something about the world that I want to lampoon or am currently trying to come to grips with. If I make a wealthy magpie of a woman obsessed with shoes and handbags, it's because consumerism is on my mind; I did this shortly after the most recent UK general election because I had a critical axe to grind and I wanted to make fun of this type of careless human being. Sometimes my characters are more personal and contain facets of trauma I want to process.
I'll make no bones about the fact that I made my current anarcho-communist Savage Skies character because I just want to let off some smash-the-fash steam. It's cathartic.
I take many of the lessons I learn this way IRL, and feel that RP has genuinely helped me become a better, smarter human. I've been in tense job interviews or nerve-wracking first dates where to fight off anxiety, I asked myself, 'What would character name do?' And then I just turn up the confidence or sex-appeal and cruise right through.
It improves my writing, and contrary to the "RP isn't writing" logic, RP has absolutely given me the tools to better myself as a writer. I lived abroad for a good few years during my childhood in a non-English speaking country, but discovering Achaea in my adolescence gave me the tools to significantly improve my English to the point where when I moved back, although my pronunciation was rubbish and my conversational style awkward, I was writing at a much higher level than the average, native English student. (When you learn English from books and medieval fantasy RP, and your favourite book is The Hobbit, you end up speaking really weird.)
Oh. And it helps that I enjoy it.
For the people interested in the derail topic on Lovecraft and cancel culture etc., here's a new thread.