They are a general indicator of the players quality. I may have two pitchers, both graded 6 with the following stats:

LOPS - ROPS
.750 .650 Player 1
.700 .700 Player 2

Player 1 will most likely end up with a higher ERA in a 20 team league as he will face predominate lefty lineups.

By the way, I know you know this Tim, I just think this is a good discussion for any new owners out there.