The same exists on some other forums (different platforms) as well as trying to like comments on websites (some news sites where comments are allowed). Many times they have a Facebook sign-in to comment. 
One reason for this is so a comment cannot garner an exorbitant number of likes from anonymous users liking a comment, leaving, returning and liking it again, or the poster liking their own comment. Another reason is because the site wants to build up their user base. 
