Not having to stop sessions of all kinds of things and restart them for no good reason.

It doesn't require it to stay up, and if things were better at retaining state across restarts I would care less, but it's a nuisance to have to log back into things, and get things back exactly how I left them.

I often have half a dozen projects up on different virtual desktops, and leaving them how they were when I worked on it last makes it easier to get back up to speed.

EDIT: I used to leave screen sessions running on servers instead, as the workaround to having to reboot my local machine. But it's nice not to need to.