To SET/ADD a new config var value:
$ heroku config:set [KEY_NAME]=[VALUE]
To DISPLAY/VIEW/GET current config var values:
$ heroku config:get [KEY_NAME]
To REMOVE/DELETE a config var value:
$ heroku config:unset [KEY_NAME]
You can use thise Heroku commands because an app will always run in multiple environments that are in production. As an app developer, you know an app can have hundreds of environment values such a an APPNAME for example.
These environment variables are used throughout the app and setting them correctly is important for the functionality of the app you are developing not only for you and but most importantly for the user base.
Heroku is using the trandional host using the .bashrc shell.