Maybe I am opening a local file? Using a local network resource? On a limited/expensive connection and don't want it auto-updating some BS? Running the browser in kiosk mode as a glorified HTML/CSS/JS GUI for a local app (apparently a thing now)?

It's really apparent when I socks proxy into a remote site (via limited 2G) to view a few kB worth of HTML admin page and firefox starts autoupdating a bunch of remote resources.