"Junk" isn't one thing — it's several different kinds of files that accumulate for different reasons, and a good Mac cache cleaner treats them as separate categories rather than one big pile. The main ones worth knowing about:
None of this is a bug or a sign something is wrong with your Mac — it's how modern software is supposed to behave. Apps cache data because reading a file from disk is much faster than rebuilding it or fetching it from the internet again; that's why apps that use a lot of images, fonts or web content tend to build up the largest caches.
Logs exist for a similar reason: when an app crashes or behaves oddly, developers (and sometimes Apple) rely on those logs to figure out what happened. macOS and most apps write them continuously in the background, whether or not you ever look at them.
The issue isn't that these files get created — it's that almost nothing on macOS goes back and deletes them once they're no longer useful. A cache from an app you used once two years ago is often still sitting in ~/Library/Caches/ today.
Not everything under ~/Library is equally disposable, and this is where a lot of manual "clean your Mac" advice gets people into trouble.
CleanMyApple scans your Mac and sorts what it finds into the categories above — caches, logs, temporary files, old installers and browser data — rather than lumping everything into one undifferentiated "junk" total.
You can open each category and see what's actually in it before deciding anything. Nothing is selected or removed automatically: you review the list, choose what to clean, and only then does CleanMyApple move those files to the Trash — not delete them permanently — so there's still a safety net if you change your mind.
It genuinely depends on how long it's been since your Mac's caches and logs were last reviewed, and how many apps you've installed and used over that time. There's no universal number that applies to every Mac.
That said, on a Mac that hasn't had its junk files reviewed in a while, it's common to find a worthwhile amount — often several gigabytes — sitting in caches, logs and old installers. The only way to know your own number is to run a scan and look at what CleanMyApple finds on your specific Mac.
Cache folders and Application Support folders can sit right next to each other in ~/Library, but they're not the same thing — cache is disposable, Application Support often holds real app data. CleanMyApple always shows you what it found, organized by category, before removing anything.
