This is the same request as Add optional box for spam flags letting you say why you think it's spam, but for Not An Answer flags, and for much the same reasons. It may well be that it would be best to do this for all flags, but this is specifically a request for NAAs.
The idea is that you'd be able to optionally say why it's NAA, and that would be shown to any mod/reviewer reviewing the flag. I don't like to burden our over-burdened mods where we can avoid it, so I'd rather be able to give a few words of explanation on an NAA and have it go to the review queue where 10k+ users can review it than flag as "Other" and require a mod to look at it.
Why this came up for me recently, just as background:
Recently, a user posted an answer to a question on Stack Overflow which said just:
You can use [name of thing], I've used it a lot with [name of technology related to the question]
I got curious about the link based on a comment (I used example.com above, but his was a real link), and it turns out to have nothing whatsoever to do with the question — the user was making a joke by linking to something completely unrelated to programming, to the question, to anything, really. (Not spam, I don't think, just a joke. Not a funny one, but then, humor is subjective.)
So I went to flag it as NAA, but of course, link-only answers aren't necessarily NAAs (though they're low-quality), and reviewers/mods may well not follow the link to find out that it's a complete non-answer.
One can leave a comment on the answer, but having a box where we can provide additional information for the NAA is useful because:
People sometimes leave unhelpful comments on NAAs spam questions, and we don't want reviewers/mods to have to take the time to read through them hoping for something useful
It allows the user flagging the NAA to do so and say why without calling attention to themselves publicly