It doesn’t have to be valid. You can fire someone because you don’t like them, or for no reason at all.

You just can’t fire them for a prohibited reason, such as race, age over 40, sex, disability etc.

Needing a reason is more of a practical problem - every person is a member of at least two protected classes (everyone has a race and sex). If you have no reason for firing a person, it’s often pretty easy for them to argue it was really based on a protected characteristic.