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](https://mathoverflow.net/help/privileges/flag-posts) 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."

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When you flag a post as low quality or should be closed, it enters a [review queue](https://meta.stackexchange.com/questions/161390/what-are-the-review-queues-and-how-do-they-work) where other users vote what happens with the post next. 

Low quality flag sends a question into [low quality posts review queue](https://mathoverflow.net/review/low-quality-posts/). For questions which enter this review queue, [2k+ users](https://mathoverflow.net/help/privileges/edit) can choose from the option Looks OK, Edit, Close (or Recommend Close). For answers the last option is Delete (or Recommend Deletion) instead. Some network-wide guidelines on what posts should be flagged as VLQ can be found here: https://meta.stackexchange.com/questions/216107/when-to-use-the-flag-low-quality

If you choose *should be closed* flag, the question enters [close votes review queue](https://mathoverflow.net/review/close/). In this review queue, [3k+ users](https://mathoverflow.net/help/privileges/close-questions) can choose between the options Edit, Leave Open, Close. (As already mentioned in the comment, for users with [privilege to case close vote](https://mathoverflow.net/help/privileges/close-questions), there is no actual difference between this type of flag and casting a close vote. A flag is automatically [converted to a close vote](https://math.meta.stackexchange.com/questions/21547/what-is-the-difference-between-voting-to-close-as-a-duplicate-and-flagging-as-a#21548). However, close vote pushes the question into the review queue, too.)

For details on when review queue is finished, see here: https://meta.stackexchange.com/questions/164288/when-is-a-post-removed-from-a-review-queue A question is removed both close votes review queue and low quality posts review queue immediately after it is closed.