It's completely objective, just as Reddit's count, your Facebook friends, or your Twitter followers. However, it helps in many ways when objectively applied evaluating a person's involvement in the community and commenting reputation.

If someone leaves a stark comment and had 10 points and 10 submissions, you can guess they have a lot to learn about this community, however if you find someone in the thousand's+ and has actively engaged in meaningful conversation, you get a better idea on the intent of the commentator in question.

Additionally, in the past when personally asking for advice (like this thread), I've used that same logic to help better understand the true value of opinions. I've also heard on two occasions from startup employers something along the lines of "..but what's their HN score?".

Oh, and after a certain amount of Karma, you get down-vote ability.