Oh all sorts of things. Mostly it's just a development instance of whatever I'm working on, which is usually something in Go these days. I use Caddy to route to various internal services based on the domain the request comes in on. I use Cloudflare to have a wildcard DNS such that I can just start using anything.vps.example.com and have the traffic show up at my machine. Also other bits and bobs like a Discourse server, photo gallery, etc.

My local firefox doesn't do anything fancy back to the VPS. It's just a browser, but it does spend a lot of time accessing the dev instances I'm running on the server. It's nice to be able to easily access my in-progress code over a proper domain, over the proper internet, with proper TLS rather than localhost. Browsers handle localhost differently than real domains and it's handy to shake out those errors early on in a project.