[ygra]

That's the usual purpose of a password manager. Freeing you from having to remember long and/or complex passwords so you can effortlessly have stronger passwords and more convenience (also different passwords for different services without having to remember them all).

Keep in mind that this is something for personal use to retrieve passwords used somewhere, not for storing passwords for users within a service (at least your confusion sounds like you might be confusing those two things).

[Anderkent]

The problem is that the 'door' is your password now - you have to remember all the different doors, or use a password manager to store them for you... But then why not just keep the actual passwords in the manager?

[ygra]

The door is just the identifier you use for retrieving a certain password. You can just use your username, or the e-mail address you used to sign up. Said identifier is not a password in that it's not secret. You can even write them down.