A couple reasons. If I am not totally familiar with whatever the original post is talking about, often the top rated couple of comments give me some good insight or jumping off points to look into it further. Again if this is something that is new to me, it gives me a hint that the poster(s) of these comments know what they are talking about as opposed to a comment rated at -3 which seems ordinary to me but maybe has a hidden agenda I have not seen. (An example of this could be--and let's pretend I know nothing about domain registrars--a comment saying, "I recommend GoDaddy because they respect wildlife on a corporate level and have family friendly advertising." I know that is a crock but if it were about a Rails hosting company or a feature of node.js I would not.)
I respect the HN community and have learned a lot here. I generally trust their judgement and I have found if a comment is rated highly, it most likely adds a lot of value to the discussion. (Unlike on reddit where the top couple comments may be a clever joke or inside reference to the community--but that is okay because I read the two sites for different purposes.)
Sometimes I disagree with the highest rated comment(s) and sometimes those comments have a bunch of replies from other HNers stating their agreement. I then see my opinion is in a minority and maybe I re-examine it or stand firm and make a comment to the contrary.
Basically it is a nice guide in my perusal of this site. It is not absolute but I like to use it as a reference.