Twilight Luna

I can't say anything that the others haven't. Some people enjoy having a version of themselves, that can be integrated into a world they enjoy. I do have one, though I regret making it, as I still haven't decided on a cutie mark - I have no talents.

let me start by saying it's the case in many fandoms, not just mlp. but why is it more noticable here? mostly because this fandom is bigger, thus having enough people to allow interactions with said OCs, and the show's worldbuilding is open enough to make it easy to add one without them being too "out there"(at least when done in moderation)

first there's the roleplaying comunity, who often make OCs to be able to explore different angles of the show's world, or pour pieces of themselves in the character they play(if you do that with a canon character, others might think you're not acting like you shoud)
then there's artist who like to experiment with new colors, new hairstyles, new things overal
people writing their own stories based in this world, even when they write about canon characters, need some OCs to fill some gaps that we just don't have info on(let's say you're writing a story in saddle arabia, we simply don't have enough canon characters to support such story, so new ones are made)
lastly, having anything non-human (ponies in this instance) makes people who imagine themselves in that world want to have a "ponysona" which means an OC that represents them, not just a character they made, so basically how they'd be/look if they were in that world (if the show had humans, they'd just imagine themselves as they are, so no OC would be made for it probably)

@The Recherche I agree that because they used Adobe Flash, the designs are pretty simplistic that any child/adult can draw a character from the show. I think that the creation of OC generating website has helped as well since as @Tacodidra said, all the characters having the same basic character model. For the non-artists, the website does everything for you. You just select what you want your character to look like. There are a bunch of these websites btw. Doing a quick search, MLP has more of these kind of sites than Sonic the Hedgehog.

Don't sell yourself short @Cash In, I'm sure you have plenty of talents.

@Lord Valtasar MLP has so much history and lore that it would be fun to create a character and explore those areas of the world either through roleplaying or writing fanfiction. There's a lot of potential for stories to be created by fans.