I think the idea is is a feable attempt at being able to tell if the person your dealing with is someone who might either take your money and run or win and auction and disappear without paying. Its better than nothing.
My other point to those that think they should get positive feedback as soon as the item is paid for ,I have to disagree. The transaction is not over until the item is received by the buyer. As a buyer it would appear to be an open and shut case that you paid timely and deserve positive feedback immediately however as a sometimes seller I can tell you from personal experience that there are alot of nutjobs and unreasonable people out there that will attempt to use the threat of negative feedback to try and extort a partial refund out of you. If you give them positive feedback immediately your at their mercy.
Even though this seldom happens it only takes one experience like this to adjust your attitude in this matter.