Kim, I'm looking back through the Community and saw your question. Sorry it was so long ago!
Subscription is useful for collaborating with external stakeholders like customers or contractors who don't have a user license.
Also, when subscription is turned on, anyone who has the proof link can subscribe to the proof and provide feedback, so you don't have to share the proof manually with particular collaborators. This is helpful when you want to get feedback from wider group of people. Does that help? Caroline Otto Technical Writer at Workfront