In the help, valid comments are referred to thusly:
Use comments to ask for more information or clarify a question or answer.
So, that I get... when the question/answer isn't clear, you ask to get more information. But there are a couple of other murkier situations.
Specifically, we will be referring to the following comment for my questions:
Why not just run dungeon world? Honestly Dungeon World and D&D have explicitly different goals when it comes to mechanics and story. – Joshua Aslan Smith, yesterday
The OP asked what I considered a very well structured, very concise question (still wondering why it was closed). He gave his background, which included the fact that he'd run Dungeon World. And he wanted to take some of the mechanics over to D&D Next.
This comment made sense to me:
Do you need to know the probabilities? Is this knowledge desired for designing the mechanic (though I'm not sure why, as it seems to be finished), or to be able to estimate probabilities when GMing? If the latter, why? – SevenSidedDie, yesterday
I personally didn't see why he needed to know the probabilities. But I suppose for balancing sake, that might be why he'd need it, which is why I wouldn't have asked... just assumed that.
But the comment in question referred him back to the system that he was trying to take the mechanic from, with a comment that the systems were meant for different things.
I guess what struck me about that was the fact that it wasn't informing him of anything that he hadn't already seen (i.e. he was trying to do this and he'd not seen the other system), and the answer to the comment wouldn't clarify the question any further.
So I'm genuinely wondering about the nature of such commentary. I've just been personally trying to limit/eliminate my comments after a recent meta question, and this caught my attention because of it.