Can someone explain what's the point of choosing a pool port with higher difficulty?

Isn't the reward (48 CREP per block) always the same?

A high pool difficulty is for high power mining equipment. What this does is lowers the bandwidth amount for both the pool and the miner. Because the difficulty is higher, the miner will find valid shares less frequently, resulting in less data transmitted from the miner to the pool.

Lower difficulties are for lower powered mining equipment, so that the miner has a chance to submit some valid shares before the next network block is found, and it has to start the hashing process over again for the new block. If no shares are submitted by a miner between blocks, that miner will get no credit for the mined block. This is why pools have several different difficulty levels.

Typically pool payouts are weighted on the number of shares per miner, while those shares are weighted on the difficulty setting, so a low difficulty share is worth less than a high difficulty share.