They're useful when you're out of position on the river and they can stop potential bluffs that you may not want to call. This way you get to price the showdown, and if you're raised big you can fold your hand. I usually 1/2 pot or 1/3 the pot if it's pretty big, but i'm not sure how one determines the standard size. I'd think that it can be calculated by the percentage of how often you think you'd be ahead in that particular situation. Bascially, if you are going to call a certain amount on the river, you might as well block it and bet it yourself.

Also, I think blocking bets can sometimes be useful if you're on a draw on the turn and you want to price your own draw, if you think that a check would make your opponent bet the pot and kill your odds. This can indicate that you may be slowplaying your hand, you'll gain a tiny amount of fold equity, in addition to getting a cheap draw and some disguise of your hand.