There are unfortunately some subreddits which are still the best place to find timely info about certain topics - for example video game subs run by the developers of the game. Lemmit is a way to subscribe to those while not giving Reddit any more “engagement” or ad views.
The bot that runs Lemmit only posts on its own instance, so users like me only see it when we subscribe explicitly (or the All view I guess, but that’s a mixed bag already. Edit: and each user could choose to ban it if they want)
I can’t speak for the admins, but my guess is that since only Lemmit communities that lemmy.world users subscribe to would be replicated here, it might not make much additional load. Also there isn’t much voting or commenting on those posts.