Complex sentence: They are commonly used in examples of apt because they are simple and ubiquitous; apt-get and apt-cache are of "important" priority in all current Debian releases, and are therefore installed in a default Debian installation.
Split into simpler sentences: They are commonly used in examples of apt because they are simple and ubiquitous. The apt package is of "important" priority in all current Debian releases, and is therefore installed in a default Debian installation.