Cache and unimportant logs

[Elmacik]

Find and repair errors results:
The subject of topic #2 is currently not stored in cache
Do you want to fix? Yes I do
Again find and repair errors. Result: Congrats, no errors. Thanks for checking.
Empty out important logs,
Again find and repair errors. Result:
The subject of topic #2 is currently not stored in cache

So if cache is something usefull, why it empties when clicked "Empty out unimportant logs"
Or if cache is something useless, then why its being used