OK, I'm doing the same here thst I usually "preach" against. So here's the TL;DR:
I find the usual "karma", reputation or any similar system basically pointless (not only here), for some very simple reasons:
* To gain karma, one has to post some popular links. You don't need to generate content, but only find it. One can posts few popular links and immediately feel validated. This one is HN specific and could easily be fixed: E.g. if links that get upvotd would not increase karma only by one or two points. Links would retain the upvotes, not users. Users themselves would gain karma through comments, or other text-based contribution. (Of course this goes against HN's busniess interest: Engagement is easier generated when it's an easy source of meaningless rewards.)
* Upvoting anonymously has the potential to turn the site into a hype-aggregator. Hive mind decides, there is no justification. This makes the usual "popular content" (hype) even more popular (overhype), while anything meaningful becomes increasingly hard to find on the internet. (There are several resources available on what makes a post/article popular, it is rarely the amount of though or critical thinking applied to writing it. And I'm not even talking about cat-videos.) Fortunately HN is not like this, not yet, but there's a lot of "debris" that gets upvoted for whatever reason. (Still, a lot of good stuff, so there is hope. Personally, I think that is due to people commenting quite actively.)
* Down-voting anonymously carries the risk to create a toxic environment. It's the simplest form of trolling: No face, no name, no reason, no justification, just press a button. But even when there is, toxicity can take over. StackOverflow suffers from the same [1].
One obvious (part-)solution would be to both of the above, to only allow up/down-voting via comments. This would still enable the more determined troll/hype-seeker, but many would be discouraged when they would actually have to voice whatever concern/enthusiasm they have about whatever they like/dislike. Obviously this is once again counterproductive in terms of total engagement, and such sites value that more than meaningful engagement, so it cannot be fixed.
The bottom-line is, the whole karma/reputation/upvote/like/clap system is a good idea in an ideal environment. It's designed to give an opportunuty to democratically decide what's allowed to float. But both the design and its users are less-than-ideal:
* The design itself does not encourage meaningful debate, it rather discourages it, although this differs from site to site. (Even where it's encouraged, the diversity of allowed opinions is curbed by the hive-mind of the site in question).
* The users themselves often use it for a way of self-validation, narcissistic self-indulgence, or just plain old attention seeking and hoarding, etc.
So that is why.
[1] http://archive.is/kTKqG