JonW already answered that well. But to provide some more background:
A flag says "Dear moderator, please do something about this, we don't want this crap around". There are cases where it is very clear what should happen, for example when somebody posts spam.
But then the question comes: what should happen to answers which are incorrect, such as somebody stating that "water freezes at +250 Celsius"? If you have the requirement that moderators keep untrue answers or bad solutions to your problem from the list of answers, you automatically need moderators who can judge the factual truth of each answer and always know the best solution for everything.
As this is an impossible task for any human being, it is not the moderators' job to remove factually incorrect answers. If the community knows that they are incorrect, it will downvote them, the low score serving as a warning to readers not to rely on the answer. Of course, this mechanism is not foolproof, because the readers are not ultimately capable of judging absolute truth either. But it tends to work for most cases.
If you receive an answer which you think is a poor solution to your problem, you can downvote it. Flagging as "Very low quality" is counterproductive, as the flag will likely be denied. Moderators even have a denial reason when handling flags, which says something like "flags should not be used for factually wrong answers".
JonW already explained what the "low quality" flag is for, but for completeness: use it when somebody posts something like "fdsfdsfdsfds" to your question, without even trying to answer. Then the flag will be valid and the moderator will delete the nonsense.