A slave/secondary authoritative DNS server for ~80 domains, a third-tier backup MX for ~20 domains with about 75 users, and a place to put some "Oh Shit" encrypted backup files that get sent there about once every 10 days.
It's a $9 VPS instead of $5, mostly because of the storage, but if I ever truly need it, it'll be worth 50x the price.
what do you do with all these domains ?
They're not all mine (though maybe too many of them are). Most of them are personal domains of mine or friends of mine, and at least half of them are little more than a landing page (or resume) and MX records pointing at gmail or proton.