What do the Request Auto-Delegation and Delegate Use of Catalog settings on the Edit User page do?
A delegate is a nominated individual who can perform tasks on another’s behalf. In Catalog we have two types of Delegate: Request Auto-Delegation and Delegate Use of Catalog.
So one way to think about this is that you're granting the named user a limited right to impersonate the user that the value is set for; either as an approver or requester. It doesn't grant them any other special privileges, such as to approve the user's requests if they don't have that already.
Further information on editing and adding users is here:
Further information on Delegating a Catalog is here:
Finally, further information on how to Delegate or Auto-Delegate a Request Pending Action can be found here: