[pvg]

aren't using flags for low-value comments

They could, and if you ask me, they should. They gum up threads and often start meta discussions about exactly how low-value they are. Many are even explicitly listed in the guidelines - snark, tropes and memes, 'broke the back button', shallow putdowns, etc. Righteously flaggable, one and all.

[freedomben]

I agree regarding snark, and pretty much anything that criticizes the person rather than the ideas. But one person's tropes and memes are often somebody else's current belief/position, especially if they are part of today's lucky 10,000[1].

What (in your opinion) is the purpose of the down-vote button?

[1]: https://xkcd.com/1053/

[pvg]

But one person's tropes and memes are often somebody else's current belief/position

In a site with the ostensible goal of 'curious conversation', that's not really good enough - it's not the job of your potential interlocutors to figure out what sincere, reasoned beliefs and positions hide behind the throwaway trope line. If you want to have a conversation, it's on you to try to converse. There are lots of other places where the trope line is fine - from the group chat with friends or colleagues to twitter. But those places work in different ways.

What (in your opinion) is the purpose of the down-vote button?

It's a way to say 'this comment is misranked'. There are lots of reasons to feel a comment is misranked - including simple disagreement.