Having an upstream can be useful for CI testing and for deployments with tools like Capistrano. Plus it allows you to pull down your code from other environments if needed and acts as a additional backup, so there are potentially lots of reasons to have one.

Because even though it's personal stuff, it's shared between many computers. For people that live in VCSs all day, it's just more convenient (history, branches, etc.) than e.g. storing it on Dropbox or similar.

Cloud access? I have a similar problem where my various machines are behind NATs/firewall (personal computer, workstation...). It could just be more convenient to put it in the cloud rather than have to poke port forwarding or VPN rules between the machines.

Well, there's the obvious benefit of tracking changes and it also provides a nice way to sync files selectively between computers.

In my case, OS X has the latest git, while Windows is stuck on 1.9, and my git folders get corrupted if I do a push on OS X and simply do a status on Windows. I guess something has changed between 1.x and 2.x but this corruption is really not something which should ever happen in a VCS. My solution is to use Gitlab instead of Dropbox only local repos.

The benefit that you don't need a git server and can have the code running on different machines! :)