Well, "Why are you doing x?" is not an answer, but it may be a very valid comment. "You should do y!" is an answer, but it is not very high quality. There is a difference. The former could very well be converted by the moderator to a comment; the latter maybe shouldn't be flagged but the user should be encouraged to add some actual context and explain their answer.
I don't know that I've ever flagged an answer as low quality, but I guess I would if I felt strongly about it, gave the user ample opportunity to improve their post (and they haven't), and the answer is not getting other down-votes (maybe because the question is off the front page, or obscure, what have you).
If we just had one reason instead of two, the moderator responding to the flag would then have to figure out which reason you meant, and they may not always see it the way you did.