I'm a subreddit mod for a decent sized esports subreddit and we have an "official" discord for our sub. Our rational for this is it provides a good medium for real time reactions to games/matches that can be shorter and more like twitch chat without having to be actual twitch chat. We noticed too that once we created the discord there was an overall quality increase in the actual posts/comments on the subreddit because most of the less thorough stuff got posted to discord instead.

It also provides a good outlet for people looking to find others to game or hangout with that share at least a few similar interests.

There's an influencer program but it's more aimed at Lan parties and other events of that nature.

I suspect that the reason you see so many Discords for subreddits is it's free, and easy. I recall few subs having IRCs which the only other option that really compared, probably because of the barrier to entry for both the user and the admin.

Reddit did launch their own chat feature. No idea if anyone uses it, I sure as hell haven't.

But the reddit chat launched when a Discord channel was already established in all the major communities. And they tried to push it too fast, when it wasn't ready to production, like their official android/ios app.