Using a sniper means you have to decide on your maximum bid and put that into the sniper. If your maximum bid wins, you win the auction, if it doesn't, you lose. All of this happens in the last 10 seconds of the auction so no time for others to add bids.
If you use eBay's system, your bid shows when you submit it. Other bidders will try to outbid you, pushing up the price. You could be tempted to increase your bid to outbid others and end up paying more than you intended!!