This is to keep auction snipers from putting in a bid in the last few seconds without an opportunity for the prior highest bidder to tespond. However, the next highest bidder's bid does not hsve to be exceeded to reset the bid countdown to 15 minutes remaining. The system still retains your proxy bid. Someone with excrutiating patience can keep increasing their bid every 14 minutes 55 seconds for example increasing their non-winning (so far) bid by the price bracket bid increment hunting for where your proxy bid tops out at. It just keeps kicking what was the highest bid by enough to be one bid increment higher than their bid if they haven't exceeded the next highest bidder's proxy bid limit.
Usually if there's a real bidding war, it'll involve 2 or 3 bidders each using the 15 minute rule to keep submitting bids until as Highlander said "There can be only ONE!"
Some auctions include BuyItNow! prices to sidestep all this, ehich is wise if the auction price is nearing, or seems to be nearing, the BuyItNow! price to bypass further bidding and 15 minute rule.
Some items are fixed price, no auction process.
Some items have "Take A Shot" option that operates like eBay Best Offer, where you can offer the seller a lower price than the item Minimum Bid price, and the offer is only visible to you and the seller where you directly negotiate.