Posting up on the block has so much to do with fighting for position, but also being smart as well and being able to read the defender. The better position you have when you catch the basketball, the higher percentage your finish is going to be. Sometimes though the defense is going to completely try to take away the post entry pass by denying the pass and playing in front of the offensive post player. At this point you don't want to keep fighting to post up, instead you are going to look for the high low pass.
The high low post drill is going to work on the proper footwork and body placement for receiving a high low pass, and then also how to finish the right way around the basket. This type of move is a must learn for post players.