Why should "for deposit only" be written on a check along with the bank account number when it is endorsed?

So that nothing else is done with when the owner isn't holding it

Because if you write “for deposit only” on the back of a check made out to you and then sign your name, then the check can only be deposited in your own account. This is called a “restrictive endorsement,” and it should prevent you or any other person from cashing the check.