Gitlab - Is there any reason not to default enable 'Allow Users to Request Access' for Projects?
It's just a safeguard for preventing unwanted user accounts from accessing projects. It's disabled by default so that you can control from the start which accounts can access a project and not have unwanted spam of requests for access.

One example I can think of, is if you only wanted users to be able to use their company email/account to access projects, and not their personal account. It could even happen by accident that someone unknowingly requests access with the wrong account.