The short answer is simply...For Fun.
Long answer is different for everyone but people like to create. OC's can just be a standalone character in someway like an imaginary friend. Or be any creative medium like film, music, animation, writing, comics ect.
The term itself being Original Character is mainly to stand for something made by yourself and imo totally Original from any other media as most I know refer to media based creations as fan characters like a Naruto fan made ninja character following that franchises world building and rules. While compared to an totally original character who is for a world you often construct and is essentially unique. Any author, film maker, game creator ect are using "ocs" if they are making something original and not associated with prior properties.
I made a lot of my characters and world because it was fun but also I wanted to see things that weren't avaliable in media of the time. Things like lgbtq+ was reaaaaallly scarce when I started my creations back in the 2000s and was something I wanted to see more of. There wasnt much in representation of Welsh folklore or culture either other than history books and some gift store mythology books, so that was another factor.
I had been lonely as a teen, art and writing had been an escape for me. I could create epics to entertain myself for 6hr car journeys and boring class lectures (back in a time mobiles only sent texts n calls and not everyone had the internet). I had to make my own entertainment, so naturally I made silly and fun things that grew with me as my interests developed and I wanted to be a writer and animator.
It feels good to have something you made that you can point to, that others can then enjoy and even participate in. A big draw here at Tapas is seeing all the different characters and stories people have created free of mass media control and pressures of Marketability. The sheer creativity is magical.