Well there are shady sellers that will get poeple to "shill" for them or do it themselves. That is where they get a different ID and keep outbidding you, then after the auction they'll say the winner backed out, would you like to buy it for your high bid? It's against eBay rules. The high bid may not have been what you would pay if they were not shilling. And there are people that will just drive your bid up for the sake of it. That's legal, but just damned annoying.
Plus bidding late leaves you open to watch for other auctions of the same item and not commit to one item only to be outbid later. or lose the opportunity to get a good Buy It Now price because you have an open bid on auction. All bids are still, valid even if it's no longer high bid, ubtil the auction ends. So you don't want to commit to 2 $500 items when you only itend to buy one.