I use Medium to write about the stuff I want to. Since I’m a bit promotion-averse, I almost exclusively use SEO to build my sites. It works really well but means I have to stay in my niches.
Medium is somewhere where I can write about SEO and the stuff I’ve learned without having to compete in a pay-to-play environment. Or write about my house rabbits. Or last night’s dinner.
Anything goes, and I like that freedom.