[rmc]

if the use case is a separate domain for serving static files, that's probably hosted on a different server/IP address anyways, right?

Not necessarily. People used to have different domains for images/css/js because browsers used to not want to download more than 2 things at the same time from the same domain name. (Back when the web was young and ugly, and bandwidth was scarce, this made sense). By having multiple domains (e.g. a.static.example.com, b.static.example.com etc.) on the same IP addres/server, you could trick browsers into downloading more in parallel and make your site seem faster. You didn't need multiple IPs for that.

Now-a-days browsers have upped their limit from 2 to something like 8 → 16 or so, so it's less of a problem.