I assume they would upvote as a sign of assent or appreciation for the comment and this would be a reward in itself.  The commentator would be rewarded(from the 38% reward pool) for their efforts in contributing to the discussion.  Upvoting also pushes comments higher in the thread to gain better visibility, so it might be the aim of upvoters to make a particular point more prominent to readers following.

Oftentimes, people will rally behind a commentator that has expressed their viewpoint.  Rather making a 'me too' post, they can just upvote?