Why does autofill not fill?

I’ve spent over an hour searching this site for the definitive word on whether autofill is supposed to actually fill in the user name and password to log into a site. I’ve seen some suggestions that developers, for security reasons, require one to click to fill in the login form.

Because it is non-intuitive that autofill doesn’t actually fill, this is a topic that should be covered by the Desktop FAQ. It would help me to hear from the developers how this function is supposed to work.

Hint: If by design, it doesn’t actually fill anything, perhaps you need to better communicate how this function is supposed to work.