Yes, but the user/server would have to be running a web server for it to be hit. Usually, that's IIS. But not every windows machine runs a web server.

Nope. WinRM or Power shell seem to use it too. Enabled by default. (Just created a 2012R2 server last night.)

Plus if you have reservations created with HTTP.SYS, your app doesn't even have to be running. A friend tried turning off IIS, but port 80 would still respond.

Nor is every machine's port 80 publicly reachable. Everyone behind a NAT won't be, for example (unless they've explicitly forwarded the port for the purposes of running such a server.)