Freeing up space · One-tap clean-up (Run Tuhi)
What does the "Clear system & log clutter" step delete?
Short answer: It removes diagnostic and crash reports and the Quick Look thumbnail cache. Your documents aren't affected and previews regenerate when needed — send any crash report you need before cleaning.
It clears diagnostic and crash reports and the Quick Look thumbnail cache. These are records and previews your Mac and your apps create automatically; removing them doesn't affect your documents, and the thumbnails are simply regenerated the next time you preview a file.
Like the other clear-up steps, it's only added to a Run Tuhi plan when there's enough there to be worth doing.
If you're sending a crash report to an app's developer, do that before running a clean-up, because the report may be removed. See why the step count changes.