In ch14s07 "Enable DNF Software Repositories", under heading "Add DNF Repositories", we have the following statement when setting up a software repository:

"Modify this file to customize parameters and to specify the location of a GPG key. Keys are stored in various locations on the remote repository site, such as http://dl.fedoraproject.org/pub/epel/RPM-GPG-KEY-EPEL-9. Administrators should download the key to a local file rather than allowing the dnf command to retrieve the key from an external source."

But we do not explain why they should do this or what benefit it provides.