apt-get clean affect installed applications?
Is it that the .deb packages there are needed for applications to run properly?
No, apt-get clean will not harm your system. The .deb packages in /var/cache/apt/archives are used by the system to install software. But once the software is installed, these .deb packages are no longer needed. You will need these .deb files only while re-installing the software. In case you do not need to re-install, remove these .deb files using apt-get clean.
Not at all.
When you install an application, apt downloads needed .deb file, and then extracts its data into your file system. The .deb file is, however, kept - so that, for example, in case you want to reinstall that application, the .deb file will have been already downloaded. That means you can safely clean that cache with
This way you can free some disk space.
From the manpage of
Note the emphasized line. You should run
The answer is no, it does no harm, not at all.