I have always been convinced that IntelliJ Idea does auto-saving of the files but now I see that there is this Ctrl+S shortcut. Is this shortcut doing something more? If no then what's the point of having it? (Maybe my conviction about auto-saving was wrong?)
Intellij has an autosave feature that activates in certain conditions, but you can turn it on or off, as desired.
The purpose of manually saving files is if you are not in a situation where the autosave will trigger but you want to make sure the file has been saved.
You can see more information about this in the documentation.