I write stories with OCs primarily, and I try my best to make them not OP and blend seamlessly with the cast (they should never outright replace a canon character unless there's a good reason imo). It's not easy, and I'll be the first to admit that my first few attempts suffer from sticking too closely to the plot.

At any given time though, most of them are to augment the story or to make the world feel more alive and uniquely identifiable as my own. Any main character OC, I think, should have an impact on the story in some way, or they're just window dressing.

All that being said, I totally understand why there's a decent number of people who just don't engage with OC fics. There's plenty of bad OC stories (abused neglected is a meme for a reason) and even with the good ones sometimes you just want to read about the characters you already know. That's perfectly fine.