Password manager can only store your nsec safely. Nsec.app can give permissioned access to other apps to your keys, and you can put nsec.app's password to your password manager to make life simpler. 

 None. Regular password manager is very good. Just means that you have to put your nsec into the client (on mobile) or in an extension (on browser) 

 I think comparing nsec.app with password manager is apples to oranges.