I own them because I like the fish. My Platydoras costatus are seldom about in night or day, yet when they do come out they look fantastic and far more impressive than any cory or tetra IMO. That's why I keep them.
And if you saw the larger pims and things that tend to hide when they come out you'd know why. Seeing a 7" fish with 10" barbels shooting around is amazing.
Another reason is for eating habits. Although you can see my Chalceus, they don't appear all that special just hanging around the tank, but dropping some food in and watching the frenzy is a site to behold.