An explanation is given in
To proceed to the next step, we need to extract the content of file, but before doing that it’s very important to know that with Oracle 11g or 12c we were allowed to set the location of our db_home directory, but starting with Oracle Database 18c we are introduced to a new concept: Image Based Installation. In Image Based Installation the Oracle Universal Installer of 18c takes the location of our setup.exe file and assigns it as its db_home location. That’s why before we start extracting the content of this file, we first need to decide where we want to create our db_home directory.
Why this point is not indicated in the official documentation?