The explanation when flaggin something as ´not an answer´ is this:
This was posted as an answer, but it does not answer the question. It should possibly be an edit, a comment, another question, or deleted altogether.
Most times when I should use the flag as 'not an answer', I choose to flag as 'Other' because I can explain why I flagged it exactly.
When I can add my expanation, there is more change a moderator understands my thoughts to flag it. So maybe it's good to add a textfield to add free text when flagging an answer as 'not an answer'.
Now the first flag option doesn't seem useful, when a user can enter free text in the 'other' option, is it?