Catching affects the general area around the WR he can attempt to make the catch. If the QB is constant in the distance he's off by a receiver, say 4 feet; the receiver w/ 30 catching will not attempt to grab it, ruling the pass incomplete, while the receiver with 68 catching will attempt to grab it, but if he fails in the grab, it counts as a drop. That's why high catch WRs encounter more drops, because their catching isn't so good enough as to always make that difficult grab. If his total # of catches is higher, then people attribute that to speed or agility, instead of catching.
I'm trying to build such a WR, recently softcapped speed leaving me to continue to improve catching. His agility and catching leads to him getting the catches, and combined with a nice 4.5 YAC, has picked up some very good first downs. I've heard some insane stuff has happened once people hit 80. You know,
http://www.youtube.com/watch?v=aojQyZMQC3o