There are several reasons for flagging answers as "very low quality". They include:
Formatting issues. Users may flag an answer that has unformatted code blocks, incorrectly formatted lists, or other similar issues. Typically, we like to encourage our users to edit to improve rather than flag an answer for moderator attention. The moderator should edit to improve the answer, if possible. If not possible, the moderator should delete after notifying the answerer of the reason for deletion. In some cases, the moderator may try to encourage the flagger to edit to improve before flagging for moderator attention in future.
The answer needed flagging, but the wrong flag was used. Users may flag an answer as "very low quality" when in fact they should have flagged it for another reason, such as "not an answer". In these cases, the moderator should dismiss the flag as helpful, and act accordingly.
However, the "very low quality" flag should not be applied to an answer if:
The answer is incorrect. It is not the job of moderators to deal with answers that are incorrect in some respect. Users should downvote wrong answers and (optionally) leave comments that describe why they are wrong. The answerer would, in the best case, update the answer to fix whatever issues it has, or delete it. We even have a badge for that.
A "very low quality" flag on an answer that is technically incorrect should be dismissed. The moderator may take into account the quality of the answer when deciding to mark the flag as "helpful" or as declined for one of the standard reasons.