Rear Rim Question

I am no expert but I have always been told that they are pretty useless as long as your bike is tuned and chain is not too old. I have never used them.

Take it off or you'll look like a newbie. Just kidding but
it really doesn't do anything. Kinda like reflectors on bikes
ridden only in daylight.

From Sheldon Brown's site:

Spoke Protector

A plastic or sheet-metal disc that fits between the cluster and the right-side spokes of a rear wheel. This is intended to prevent the derailer from getting caught in the spokes, which can cause very extensive/expensive damage/destruction to the wheel, the derailer, and the frame.
A spoke protector is not a necessity on a bike that is well treated, because the derailer can't go into the spokes if it's properly adjusted and if it is not bent. Bicycles which are subjected to rough handling, however, are prone to getting the rear derailer bashed in, and in such a case, the spoke protector can prevent very serious damage.

So - basically of your bike is properly tuned you shouldn't need it but not having one when you need it can be either a needless expense or an opportunity for much needed upgrades.