The purpose of the 'not an answer' flag is well summarised in the meta-discussion [Flagging as 'not an answer'][1]. I reproduce it here for convenience.

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The "not an answer" flag is for the following situations:

- The OP...
 - needed to update the question with new information, but posted the new information as an answer.
 - wanted to reply to an existing comment or answer, but posted the reply as an answer.
 - posted a "resolution answer" saying something along the lines of "Joe's answer worked for me"
- A user...
 - wants to reply to the OP, an answerer or a commenter, but doesn't have enough rep, and instead of thinking "maybe there's a reason I'm not allowed to post comments," ignores the help text about what an answer is.
 - posts to say "I'm having this problem too, does anyone have a solution yet?"
 - has a related issue and isn't aware of the "Ask Question" button.

These are common situations for new SO users who may be confused by the reputation, editing and/or commenting systems. They may be used to forums where it's normal to add a new post underneath the existing posts, and blindly click the "Post Your Answer" button.

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So:

 1. If the answer really isn't an answer, but something else entirely (see above), flag it.
 2. If an answer is technically inaccurate, wrong, or just doesn't answer the question, **downvote it**.

  [1]: http://meta.stackexchange.com/a/81392/132073