because by bidding early you are letting people know that there are others watching it with an intention of bidding to win, therefore making it more desirable, thus making it worth more in some peoples eyes and so making them willing to bid higher than they would have normally done, I generally find that having a final price in mind at the start of the auction and bidding late gets the goods at a lower price, if it goes higher than your top price before you get to bid then it wasn't meant to be yours, but never mind there'll be another one along eventually.