Ok I know this is old but I do this constantly. I almost never bid on an item until the very very end.

It's often the only way to win items when you're on a budget. For example, you bid $30 5 days before the item ends, I'm guessing that if it goes up to $31 with 7 hours left you might say "hmmm maybe another $5-$10 dollars wouldn't hurt" and you place another bid for $40. Then I would have to say "hmmm maybe I think it's worth $45". If I bid $31 in the last moment I'm less likely to get outbid by the top bidder and less tempted to go higher than I originally wanted.

Proxy bids mean little, so many people get tempted to go a littttle higher in the heat of the moment. If you wait to the end you're less likely to drive things up. It's why some items can go up $100 in the last 30 minutes, you've got like 30 people just watching and waiting. I've lost bids by a dollar in the last seconds a bunch of times, I've also won some by a dollar. I don't think goodwill cares enough to hike things up in that way, they already do that with their dumb shipping and handling charges.