Why rate comments?

Why do we need a rating system for comments? Aren't people more inclined to read ones with negative ratings, because they want to know why they're so low? So really, the ratings seem to be giving the spam comments more attention than actual helpful comments.

Why is that?

I have no feeling either way on this debate I like the comment rating system but i do look at the ones below -2. However they are needed to make an effect on the the user itself. I think there should be a user comments rating by adding all the comment scores together. This would make less spammy comment like "first comment" and swearing just to try to keep that score high.

Do you understand what im saying?

yes, and its very true!

...not just that one comment ratings but every comment that he/she has made put together. for example:
one comment gets -2
another comment gets 4
4+ -2=2
so that persons overall comment rating is 2

thanks

Sum? Better average.