Well, one reason to recommend after the AP is because that’s a standardized curriculum, so if the school has covered it at all reasonably we know what’s been covered. There are sample tests in the book the College Board puts out and quite a lot of stuff on their site on line was well. So it’s easy enough to figure out if you’ve got the material covered.
There was one story recently that Caltech did not want to see 8th grade scores, but unlike the regular SATs I don’t think subject test scores get repressed.