On macOS, dragging an app to the Trash removes the app bundle itself — but very little else. Most apps also write files to several folders in your user Library: ~/Library/Application Support/ for settings and data, ~/Library/Caches/ for temporary files, ~/Library/Preferences/ for saved settings, and sometimes ~/Library/Containers/ for sandboxed app data.
None of those folders are touched when you delete the app. They stay exactly where they were, except now there's no app left to use them. That's what an app leftover is: a folder or file with no application attached to it anymore.
A single leftover folder is rarely a problem on its own. The issue is time. Trial software you tried once and forgot about, one-off utilities you used for a single task, and old versions of apps you later reinstalled through a different method (an app store version replacing a direct download, for example) all leave something behind.
None of this shows up as one big alert — it's dozens or hundreds of small orphaned folders scattered across ~/Library, each one easy to ignore individually, but adding up to real storage over years of installing and removing apps.
In principle, checking whether a folder in ~/Library is a genuine leftover is simple: does the matching app still exist in /Applications? In practice, it's harder than it sounds. Folder names don't always match app names exactly — a developer's bundle identifier, company name, or an old product name can end up as the folder name instead of what you see in Finder.
That mismatch is exactly why manually auditing ~/Library is unreliable. You can end up deleting old application files on Mac that still belong to something you use daily, or leaving genuine leftovers behind because the folder name didn't look familiar enough to investigate.
CleanMyApple's leftover finder scans the folders apps commonly use — Application Support, Caches, Preferences and Containers — and checks which of them no longer correspond to an app installed on your Mac. The results are presented as a list for you to review, grouped by the app they once belonged to where that can be determined.
This is different from uninstalling an app you're removing right now, which is what CleanMyApple's app uninstaller handles. The leftover finder is specifically for cleaning up after uninstalls that already happened — sometimes years ago — where the app is long gone but its files never were.
Some folders in ~/Library that look unused are still legitimate: system frameworks store shared data there, browser extensions keep their own folders, and some apps deliberately store data outside their main app bundle so it survives reinstalls. A folder can look orphaned and still be in active use.
This is exactly why deleting things from ~/Library by hand carries real risk, and why any tool that touches this folder should let you review candidates rather than delete on sight.
Never delete a folder in ~/Library unless you can confidently identify what it belongs to. CleanMyApple only ever suggests leftover files as candidates for review — it never deletes anything automatically, and removed files go to the Trash rather than being permanently deleted.
