Cognito is a popular auth service that's used by many people who want to launch an MVP on AWS, for example. You would not keep the password yourself at that poi

Cognito is a popular auth service that's used by many people who want to launch an MVP on AWS, for example. You would not keep the password yourself at that point. Later on you might realize that you've bound your user ID to email and that you need to use UUID, where email is just a contact address. You might even want to have users have multiple accounts that use the same email address. You might decide to bind the user's account to their phone number instead. It's not about One True Strategy, but about looking further into what you will need in the future. You might decide not to develop something right away, but you can avoid much refactoring later by making a smart decision now that did not involve any more work than the less smart decision.