Need a third hand to hold the other end of something up in the air, or level with a beam, while you are struggling to get the opposite end hammered in at the correct position.
You could just drop a couple of nails into the top of the floating piece and bend them over so that they hang from the other board that it will eventually be nailed to. This can be especially useful when trying to attach long deck joists to the sides of beams, and you just can't reach even to the middle of the joist to hold everything up.