I did once in 2004 I think.
It wasn’t a great story. On the other hand it wasn’t simply ‘filling up with words’. If anything I gave myself too much story and had to just sort of cram it into the limit.
When I read it back 6 months later some bits I’d written quite well and I gave a go at editing it to make it better. Probably lost about 10K words and added 30K in that process? The result was still too bad to share with anyone. It was hacky as hell but the final 70K version was at least a story. TBH having read some of the crap people self-publish via Amazon now maybe I should have kept it?
So yeah, if you fancy being a writer I think you can learn something about how hard it might be and also how useful just writing something, anything, can be in giving you stuff you can then edit.