Or even if you just want to improve your writing in general. Before you ask, yes I do roleplay- on Tumblr, I run two different RP blogs for two different muses (the character you write/perform as). You could say I like to spend a good chunk of my free time on my laptop on social media then. But even if roleplaying does sound like a nerdy hobby, it's literally the reason I feel that my writing has improved so much- unlike other writing hobbies, roleplaying isn't just a writing, it's also an act of socializing and of storytelling.
1. You get better at writing in general.
And unintentionally too. When you write a response you figure out quickly what makes up a good response, what will make your partner want to respond back. A good roleplay reply, although crystallized into a few paragraph teaches you the essentials of good narrative- action, dialogue, grammar, strong characterization. At least for me, learning the essentials is really a matter of doing, not reading through a textbook. The best part is that since it's a partner activity you don't have to carry the story alone- you write it up with other people. And if you love your character enough, you'll want to do them justice with every response you write.
2. You make friends. Writer friends.
They may not necessarily be published authors, but some of the best writers I've met actually came from roleplaying with them on Tumblr. And when I became friends with them, I actually studied their writing styles and tried to incorporate some of their style into my own. For example, there's this one roleplayer who uses pop culture references in their character's dialogue- their character is absolutely hilarious to read because of the dialogue used. That and I've met some of the sweetest people in different parts of the world thanks to Tumblr roleplay.
3. You start to form healthy writing habits.
Have you ever looked at a blank sheet of paper and just freaked out? I do- whenever I start a long writing assignment, it intimidates me because I will think about how many pages I have to fill up. A typical roleplay response, on the other hand, is usually just one to two paragraphs, which is a lot less intimidating. And if you like your muse a lot, it's not hard to start writing either and eventually, that translated to everything else I started writing. I started roleplaying on Tumblr when I was sixteen years old. At the time, I had also been taking a class with a teacher who was 'guaranteed to make my writing better'. Out of the two, honestly, RPing on a daily basis made me less scared of putting words down on paper.
This should be a given. Writing can be a lonely career. Writing prose can be tedious and at times boring, but when you start feeling lonely and burnt out, that's where roleplaying can help. It doesn't just give you a chance to improve and show off your writing skills, it also gives you a community of like-minded people as well as a support network. That and there's nothing better than logging into your roleplay account and seeing that a partner has responded to your roleplay response or that someone new has responded to your open starter.