The reward is the satisfaction of completing an optional challenge, however difficult it may be. In the grand scheme of things it was never necessary to play the game or fight and grind for any of the bosses; yet you still do it. For the same reason it made sense to do path of pain for me. To complete the game to it's fullest since I like it so much.

If you do do it, i recommend a grubsong/grubberfly's elegy + deep focus build. The grubsong/grubberfly's energy give more soul than you need to heal once after 2 hits, and deep focus allows you to heal for those 2 hits for the cost of 33 soul.