Purpose - The purpose of adding Steem Power to musings is simple. Musings can only reward quality content creators on its platform if it has sufficient Steem Power. Thus the purpose of giving Musings Steem Power is that it retains good authors by rewarding them adequately for their work. Without Steem Power it would be another copy-paste of Quora and when you already have one flourishing network in the form of Quora why would people bother to look for musings ?
Advantage - With Steem Power in play Musings beats its competitors in every possible way. Because of Steem Power musings will attract good authors and the quality content created by them will further attract more visitors The traffic on this site will gradually increase with time.
When you build a wall, you lay bricks on concrete layers to hold the bricks together. Steem Power is the Concrete that binds all the bricks(us) in order to build a massive wall.