A. Get link juice
B. Get Traffic back to your site
C. Get Authority in another community.
If you run a website with open comments then you have felt the pain in the keester of having to delete stupid commentary.
And I am not just talking about the obvious comments like, "great one". I am talking about the guy who writes that is wonderful good going.
This person is missing out on the authority juice that can come from the readership in this community who will be so delighted and so intrigued by his commentary. That they will want to visit his brand. That authority juice is human. Better than anything Google can ever algo out.