CodeIgniter3: Why would $_SERVER['CI_ENV'] ever be set in the first place?

As Oliver described; it's a special-use case for multiple environments. Splitting out the development, testing & production by means of .htaccess before it even gets to the code. To configure this:

Development (Localhost)

<IfModule mod_env.c>
    SetEnv CI_ENV development
</IfModule>

Testing (Your Local Server)

<IfModule mod_env.c>
    SetEnv CI_ENV testing
</IfModule>

Production (Remote Server)

<IfModule mod_env.c>
    SetEnv CI_ENV production
</IfModule>

You're right in thinking it won't ever change unless there's some manual intervention. There's not much documentation in regards to this:

"This server variable can be set in your .htaccess file, or Apache config using SetEnv. Alternative methods are available for nginx and other servers, or you can remove this logic entirely and set the constant based on the server’s IP address."

Source: Using the Environment Constant

Just in case you are using nginx, here is the configuration you have to add inside vhosts configuration:

  server {

        location ~ \.php$ {
            fastcgi_param CI_ENV production;
        }

save and run nginx syntax check (just to avoid you from cursing me):

nginx -t

if you having issues or you cant find the parameter, you can follow the answer below: Nginx variables similar to SetEnv in Apache?