Lock and login are not the same.
Login = Account access (you account includes your vault, which is a password storage area).
Lock = Mechanism of your vault to protect your password after certain idle time.
You can be logged in (your account, in the browser extension) and locked out (password won’t auto-fill). Then you need to unlock the vault in the extension, not in the site.
There’s a lot of people who simply leaves their assets (computers/phones/tablets/watches/etc) unlocked even if they are physically away from them. Locking is to mitigate such oversights.
You can increase the locking time or even disable it (if we’re talking about the extension). However, is generally not recommended unless of course you’re fully aware that if anyone gains access to your browser, your passwords are compromised.