You want to set a configuration option.
But you don't want this value to be stored as part of your permanent configuration.
Let's say you want to set the session driver to use the array driver.
Now, for the remainer of the request, any time
Config::get('session.driver') is called, the value
array will be returned.
The only affects the current request.
If you want to change a configuration value permanently, so all future requests will retrieve the same value, you must edit the appropriate configuration file.