To use the combination lock example, if someone knew about the trick and wanted to find you combination,
they could wait till after you were the last one to use the locker, then, given even that you put it to 15, so they don’t immediately get the last digit of your password, they only have to try on 10-20 (Locks have some leeway, so you don’t have to get it exactly right) different occasions to get it open. If I actually wanted to get into your locker, it wouldn’t be that hard to.
They still only get one shot per time you leave your locker, but that’s not that large an obstacle.