Using a password manager is great! But there are still various ways people can get your password; e.g. if they break into the email you use for password-resets.
If you also use a physical security key (and it’s properly implemented), an attacker also has to figure out how to steal or forge that physical object. This makes it much harder to attack protected accounts.
You generally wouldn’t use something like this for every account, but you might for a few key ones (like your LastPass account and the email address you use for password resets) that are particularly important to you.