BIN (bin) = Buy It Now price
If a buyer can use the buy it now, the buyer can end the auction. Once the buyer has exercised the buy it now option, the item is sold, so there is no auction to continue.
It's just different ways of doing it. I believe ebay invalidates the bin option once the bidding meets/exceeds that price. At that point bidders have lost the chance to use bin to complete the sale. While the bidding is still below the bin, bidders must decide if they are willing to pay the bin price to be sure they won't pay more. Or, if they don't believe bidding will go that high, then bid under bin and wait it out. Buyers will pay a bin higher than the current bid price if it's worth it to them, and they believe other bidders are likely to bid the price up anyway. Of course the seller is risking missing a possible higher bid price, but on the other hand they can sell the item more quickly by offering buyers a chance to get the item now. That's what some auctions do.