There are certain sites which terminate your sessions after a while. For example, banking sites or most government portals. In such situations, the auto fill function is very handy.

Like the other commenter said, I use it for sites that tend to sign me out after a few hours. I also use it for work things that sign out every session.

I auto wipe cookies all on every browser close so that gets more use.

Way way late to this, but I'll also say: Firefox and other privacy-focused browsers have an option to delete all of your browsing history and cookies when you close the browser, which also logs you out of anything you were using. It's a good practice if you're being mindful of how much tracking data you are letting be collected from you.