Four things accumulate over time. Three of them may go, one only with care.
Set-aside content folders
Switching the content folder does not lose the previous one: it is set aside rather than deleted, and the Snapshots page lists it. From there it can be put back or deleted for good (the bin on its row).
That is the only copy
On deleting, QuartzControl says so clearly: a set-aside folder is not included in the snapshots, and it cannot be undone.
And once more, because it is the most common misunderstanding: this is not a backup of your notes. If the content folder was a link to a vault, nothing was ever copied — only the link was recorded.
Old snapshots
They live in the project and grow with it. Deletable individually, via the bin on the tile next to Compare; the question about it is short, because deleting a snapshot changes nothing about today’s state.
If you need space, delete the oldest and keep the ones with a name — those were taken for a reason.
Built plugin files
Under Plugins → Maintenance: Remove orphaned plugin folders deletes whatever no configuration points at any more. The configuration itself stays unchanged — nothing the site needs is lost.
The counterpart beside it, Restore from quartz.lock.json, fetches them back.
The theme cache
In the settings under Data & maintenance: the labels of the theme options, fetched from GitHub once and kept. Clear cache forces a fresh load.
The only reason for it is that a theme changed its descriptions and QuartzControl still shows the old ones. The cache takes up hardly any space; the card names the number of entries and the size.
The build folder
It is emptied and refilled completely on every build — so there is nothing to tidy there. Anyone wanting it gone for good deletes it; the next build creates it again.