What are the original characters people request to draw for?

I am in several subreddits where people request something to be drawn, and I often see requests to “draw my OC such and such”. Are they for pen and paper RPGS? For fanfics maybe? I feel like I’m old and missing some cultural context.

Sorry if this question doesn’t fit the subreddit, but I thought if anyone, the artists who come across these requests might know a thing or two about them.