GPG is commonly used to send encrypted email back and forth between two or more parties. If you have any kind of secret or proprietary information, using email encryption is the best way to make sure the content is readable only by the people you trust.
But when not used for email, GPG can be used to encrypt individual files or archived files. If you don’t want to set up encryption in your email client, or if you don’t have the privilege, you can still encrypt individual files and send them as attachments.