First of all, the only way you get ??? players is to not see a full 500 prospects. With 20 million High School (on one of my teams), I usually see only around 200 prospects with projections. With 20 million College scouting, I almost always see the full 500. But there are other owners who budget less than 20 mil College, so they'll see some ??? college players.
In the High School draft, I now intentionally draft ??? players from about round 10 onward. Somewhere around my 120th prospect, I start seeing players who might never get out of A ball, even if they DiTR they'll still be absolute crap. So I throw them down to the bottom of the pile, after all the ??? guys. I'd rather take a lottery ticket than just waste a mid-round signing bonus.
College players whose drafting owners don't see projections might leave them unsigned. They can show up in the next season's minor league free agent pool. So I always look at the 23-year-olds, see if a good one slipped through.