What does the “Custom CSS” field actually change?

In the plugin if you click newsletters>templates, and in the menu just under the subscribe field, “Settings” it brings up a field that says “Custom CSS.”

At the bottom of that field it says “CSS added to the main newsletter CSS.” I cannot figure out what this CSS actually does though. I don’t see any changes reflected in my template. I figure I’m just missing something or being exceptionally dense!