People use other managers for many reasons: storing passwords (and other secrets) which aren't used on a site, using them on different browsers (say, Safari on the desktop and Chrome on mobile) and lack of trust on the browser's password manager.
Also, for a long time, browsers didn't save passwords with forms marked with autocomplete=off.
To generate random, strong passwords. Also not to be locked into a browser. Better actual password management (e.g. last changed). Tags.
A canary of chrome did have the ability to generate random passwords, but password management in chrome is still a pain IMO. Not sure about FF, but a quick google suggests it doesn't generate random passwords automatically.
People use other managers for many reasons: storing passwords (and other secrets) which aren't used on a site, using them on different browsers (say, Safari on the desktop and Chrome on mobile) and lack of trust on the browser's password manager.
Also, for a long time, browsers didn't save passwords with forms marked with autocomplete=off.