Why do I need to log out of the account (logout, sign out)?

Question

OAuth

For the first time I asked myself the question: why, in fact, do you need to log out of the account?
Of course, in the case of online banking and other at least some serious applications, the question is not worth it.
But here I am completing a service for make-up artists, hairdressers, manicurists, etc. And the login is exclusively through facebook. Those. even if the user logs out of the account, then to log in from the same device, you just need to click the "sign in with facebook" button. The element of protection, it turns out, disappears.
Why then do you need to log out of the account at all? Maybe, well, him?
PS: has anyone come across websites/services without the ability to logout?

Answer 1

An exit is needed so that two makeup artists do not fight because they logged into someone else's account. They also come out of Facebook, yeah.