[David McLaughlin]

In addition to what red said, Having paid for said service (or member cards, etc) you are more apt so shop only them exclusively. Yes, many people do not use the services places offer (I now work at Barnes and Noble and have worked at Home Depot and several other retail places that have cards or memberships of some sort) and that does give them some extra money, but what they are looking for is to keep the customer from shopping around.