An Ad-Hoc Approver is NOT a required role for your department to assign in your workflow. This is simply a role that you can assign on a case by case basis if you are ever in need of an extra approval or set of eyes on a particular order.
The example that we typically give is:
Perhaps you have found funds in your budget (go you!) to order all new computers for your department. Besides your usual set of approvals, you really want your incredible IT team to take a look at the order to make sure that everything you are ordering is top notch and right for your team before you buy. You don't need them to look at every order that goes through your department, but this one time-specialty order could really use their expertise. Using the Ad-Hoc feature means that you can simply add in members from your IT team to check out your order and approve it or make suggestions before you finalize the order. This role can be added in the GMC.