The purpose of separate, secure passwords is because otherwise all it takes is 1 website out there to be insecure to compromise all your logins. If your computer is compromised you already lost, regardless of whether you use a password manager or not.

If a person accesses your phone or laptop, he or she will not necessarily have access to all your credentials for various services and for other devices. But if all this info is stored in one place, in a password manager, the scope of the potential data breach suddenly grows.

They don't suddenly have access, if your password manager is secure and your master password is strong.

That's one of the nice things about password managers. You reduce the number of potential points of failure from many to one. Why is this good? Think of Thermopylae. You increase the stakes, but you also dramatically improve your ability to fight back.

The purpose of memorizing the passwords is a) so you always have access to them b) so they are safe

The idea is that password managers should meet both these goals as well.