How do blitz games help with your opening theory? I thought they would not help because you might tend to make a lot of mistakes during blitz or is this the reason, to keep playing them over and over until you make the right moves?
Here are a few reasons for why players use blitz games to prep for openings.
There are only a limited number of standard games (normal speed 20-30 moves/hr) that one can play. However, you can run through a lot more blitzes in one evening.
If you want to try to familiarize yourself with the positions that result from some variation, you can play a few blitzes to get the feel for it.
Blitz games serve to remind you of the standard motifs that go along with certain openings. Also, you get to see how opponents not well-versed in the openings handle your moves.
Some players try out new openings (to their repertoire) in blitzes before trying them out in tournaments.
I have encountered blitz opponents who will routinely resign after the first 20 moves or so (no matter whether or not they had a better position) just so that they can focus on the opening again. (They don't care for their blitz ratings.)
So in sum, blitzes can be of good help to the amateur, but it should not be the only preparation for learning openings.