It seems that we are mostly talking here about close flags and very low quality flags - since these are the two types of flags which seem reasonable in the case of low quality questions (whichever interpretation of the phrase low quality you choose.)
Since the OPs main concern seems to be whether the flag adds work for moderators, it is worth mentioning that these types of flags in the ideal scenario are actually not handled by mods but by other members of the community. The help on flags explicitly says: "Many flags are handled by other members of the site like yourself, who've earned review privileges. Close flags, Not an Answer, and Very Low Quality are primarily handled this way."
When you flag a post as low quality or should be closed, it enters a review queue where other users vote what happens with the post next.
Low quality flag sends a question into low quality posts review queue. For questions which enter this review queue, 2k+ users can choose from the option Looks OK, Edit, Close (or Recommend Close). For answers the last option is Delete (or Recommend Deletion) instead.
If you choose should be closed flag, the question enters close votes review queue. In this review queue, 3k+ users can choose between the options Edit, Leave Open, Close. (As already mentioned in the comment, for users with privilege to case close vote, there is no actual difference between this type of flag and casting a close vote. A flag is automatically converted to a close vote. However, close vote pushes the question into the review queue, too.)
For details on when review queue is finished, see here: When is a review task "completed" or "invalidated"? A question is removed both close votes review queue and low quality posts review queue immediately after it is closed (or deleted).
Some network-wide guidelines on what posts should be flagged as VLQ can be found here: When to use the flag “low quality”? The official desritiption of the low quality flag is: "This question/answer has severe formatting or content problems. This question/answer is unlikely to be salvageable through editing, and might need to be removed."