Some are also small servers run purely for the hell of it; if you keep the user count low it really doesn’t cost much to keep one running. These instances are of course more vulnerable to their owner being hit by a bus or whatever and not having shared their keys with anyone. Less so once account migration lands, and everyone’s gotten off their butts and upgraded their instances to that version.

(My instance, by example, currently has about 150 accounts, closed registration, and costs about twenty bucks a month to run. It’s also a few versions behind because I’ve been busy with other things.)