I like to see that a comment I wrote is marked by somebody as useful. In the overall of my make comments it serves as a feedback (and it's nice to read something equivalent to "good point"). But apart of this, what consequences has it, in reputation or something? I've searched in this part of the site, math. and math.meta, and I didn't find anything more than the obvious, that you can click to increase that index.
Voting on comments has no effect on reputation.
Once there are too much comments, the ones with lower scores will be hidden by default (you have to click 'Show more comments...' then), so upvoting the comment will increase its visibility.
Also, comments with five upvotes count towards the Pundit badge.