If I'm already using Firefox on Mac, Windows & iPhone, what advantage does this bring over just opening the Firefox app on my iPhone and looking in Settings > Saved logins? Seems to me to be exactly the same, am I missing anything?
It's faster. It syncs. It's safer than firefox passwords manager. It unlocks integrates better with the OS (unlock with fingerprints or face). You can use it for something else than websites.
However, it's still nowhere as convenient as lastpass.