Most basketball coaches have their first big man up the floor in transition run right down the center of the floor to the front of the rim. This terminology is called "rim running" or "running the pipe" (there are some other terms for it as well). The big man wants to run right to the front of the rim and try to get a catch and finish before the defense gets back.
A lot of times though the point guard will be out around half court when the post reaches the front of the rim. So the pass is going to be over top of the defense or fired in from the top of the key/wing area. This can be a difficult pass for the big man to catch and finish on the run. That is why this drill is so important for big men to do. Basketball drills like this are game specific movements that are going to be used during the course of a game.