Confused about Powerprep

PowerPrep is a GMAC product; it's the older version of GMATPrep. There's a difference between the two, however- GMATPrep clearly has a much larger pool of very difficult questions. This means that on GMATPrep, you can get a lot of questions wrong and still get a very good score, just as long as you're only answering the supremely difficult questions incorrectly. On PowerPrep, the hardest questions seem to be only medium-difficult level, so if you get a couple of these wrong, the test becomes convinced that you're not a top (50/51 level) scorer. The underlying scoring algorithm should be more or less the same, but the question pool is quite different.

When I took the powerprep tests I found it useless as most of the questions I had already seen before in OG.

I honestly wouldn't worry about it. The Powerprep tests are too old to be relevant anymore, and the scoring algorithm doesn't match the current GMAT. I wouldn't bother taking them. If you've finished with the GMATPrep tests, then take Princeton Review tests...they're the next closest tests.