The other people using the app wont know what I've bid either until it's too late so they wont be able to react if their snipe bid is lower than mine. The psychology of auctions is that people get caught up in the bidding process and have to make quick decisions on whether to increase their maximum bid above what they were originally comfortable with or miss out on the item. If you don't reveal your bid you don't give other bidders that chance to change their bid after yours (whether they are using a sniping app or not).
Using a sniper app isn't pointless because it puts you at an advantage over anyone who doesn't use a sniper (and there are plenty who don't) and you are also therefore not at a disadvantage to other snipers.
Bidding earlier (manually) than you need to on an ebay auction is completely pointless - all you are doing is pushing the price up and unless you've bid quite a way over the odds you are going to get outbid on most occasions.