They generate extremely strong passwords with very high entropy. Basically if you can remember a password, then it's guessable. That's why you have a master passphrase not password for the vault itself. a long sentence - puppy shark tree eats dolphin shoes! or something like that. Since that passphrase never leaves your computer, short of having a keylogger installed (and then it's game over anyway), and each site has it's own unique strong password , if one site is compromised, then the bad guys can't cascade across all the others.Only if you use it for every site? Consider the options... maybe you just use it for all the unimportant ones? But then... how do you remember the important ones? What's your solution?
Note that your email account is the most important to secure with an unguessable, unrememberable password because it's used to unlock/recover all the other accounts.