On this question I flagged the following comment for being an Answer in comments:
Actually, I don´t agree with most of the answers. This is the basic principle of a handgrenade. Most SAM´s are working on the same principle. I don´t see why a space-anti-ship-missile should not do this in an up scaled nuclear version.
This flag was declined. Why?
It is my understanding that
If you disagree with answers, you should downvote and optionally create an opposing answer
I can't downvote this comment
It is certainly true that the primary (and, idealy, only) purpose for comments is to request clarification for either a question or an answer.
Reality is somewhat more opaque.
Comments are used for all kinds of things. The least valuable are playful banter and flat-out arguing. The most valuable are for questions and insight.
And insight is no small thing.
The comment you're referencing could be interpreted as the basis for an answer, but frankly, it isn't a direct answer. It is insight, which the OP may or may not use to improve their question.
Nevertheless, you will occasionally see comments used for a complete answer. I've never personally flagged a comment. I consider them part of the discussion (and only an actual issue of some or all of that conversation becomes so long that it detracts from the question or answer, that's what chat is for). However, on rare occasion I've left a comment that points out another is an answer, a good one, and maybe the comment author should post it as such. Sometimes they do.
Sometimes they don't.
Why? Sometimes the commentor hasn't the time to create a full answer. A lot of us are having fun on this site in the small hours between work, family, and other comittments. We have what we hope is valuable insight, and occasionally that's all we have the time to offer.
Sometimes the commentor wants the OP to be aware of an alternative, but simply doesn't want the rep, or would prefer not to detract from existing answers.
Sometimes the commentor is providing a complete answer that's too short for the answer section, leaving only comments as an option.
Is it important? Frankly, no.
This SE site is much better at avoiding answers-in-comments than others (you should visit English.SE and ELL.SE, where answers are regularly posted to comments before a first answer is even posted). But, to be honest, the interaction is just as valuable to the OP as the answers themselves.
But, this perception doesn't come without an obligation
And it's one that I must admit I have occasionally failed. Comments should always be civil, courteous, and supportive. Too often commentors take serious swipes at questions and answers alike, as if the author of said Qs and As should magically have the education, experience, and insight those who swipe have themselves. I've only occasionally slapped someone on the hand for being, shall we say, just a tad too blunt - but only occasionally, because (a) I've thrown a few of those stones myself and (b) I frankly don't want to jeopardize what good reputation I may have simply to reprimand the behavior of another. Perhaps (b) is very selfish of me, but I'm human.
I would like to point out that you can flag a comment for being rude. Perhaps I should take more advantage of that. Gratefully, I don't think rudness is rampant on this site.
Anyway, in the long run...
I suggest that comment flagging should err on the side of mercy. Occasionally reserved for the truly obnoxious, but perhaps more often for cleaning up very lengthy comment chains by having the mods clean out the playful banter and not-on-point arguments.
And even that goes against my personal freedom-of-speech sentiment. Yes, SE is a private organization and that freedom is not intrinsically protected, but it's a part of me nonetheless, and who am I to be judge and jury over a comment?
Conclusion
Don't worry about it. If suffering imperfect comments is the worst thing that happens to you in your life, you're doing great.
