Why does the catcher pass a pitched ball to umpire if it hits the ground?

Because if it has scuffs on it, it is considered a "bad" ball and it cannot be put back in play. That's why they check it & the umpire sometimes puts it back in play. ...and the reason a scuffed ball should not be used is that you can get more 'action' out of your pitches on a scuffed ball. The pitcher can get a better grip on a scuffed ball, and the scuffs provide more 'bite' on the air allowing a curve to have more snap or a sinker to drop more dramatically, for example.