Say there's a 2/2 Boros Guildmage making your life hard. During your upkeep, before you draw your card for the turn, Darkblast it to give it -1/-1. During your draw step, instead of drawing your card for the turn, dredge Darkblast back into your hand. The Boros Guildmage is still 1/1. During your main phase, Darkblast it again and wave bye-bye.
If you have a way to draw multiple cards per turn, a single copy of Darkblast can take down creatures even bigger than 2/2, if you're willing to dredge away hunks of your library!