This is pure speculation, but I think it decreases the chances of legendary engrams decrypting into rare items (which drives me crazy), and increases the chances of your engrams of any type upgrading to a higher level (like an uncommon turning into a rare, etc.) You also get a Legendary package of sorts from the post master when you reach a new cryptarch level, which is also fun. Another thing I've noticed is that you gain more experience from decrypting engrams found in the field, as opposed to spam buying the ones he has in stock.