Question

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

Answer 1

to make sure your money is deposited into the correct account

Answer 2

It is important to write "for deposit only" at the back side of a check so that the banks only deposit them. You also sign at the back of check.

These specified checks cannot be cashed by anyone. This is called restrictive endorsement.

This will further prevent any sort of illegal activities of any sort.

If the check is lost, nobody else can cash it or do anything with it.