As someone mentioned, my reasons are:
1) avoid being bugged by un-gentleman buyer who calls you to ask this and that after getting the car (when was this last changed, which spare part number should I get to replace this/that), or complain this and that.

2) avoid the hassle to deal with buyers who are not serious, or expect to buy at much lower price that what a dealer charges for similar car (without taking into account of football money they need to pay to dealers).