They're officially called "wheel protectors". They're needed if and only if the rear derailleur stop is not properly adjusted and the derailleur shifts the chain off the largest cog into the spokes. If that happens, either the wheel stops instantly, or there is a loud crunching noise and then spokes start to break.

I personally call them "Dork Discs" and have removed them from all my bikes. If you properly adjust your derailleur, you don't need them.

If you're a manufacturer, then you'll insist on having them on for liability and warranty purposes.

Really, it's your call, if you remove them, then you assume the responsibility of your actions. I don't like the way they look and they do nothing for me, so I removed mine! I think the wheel looks "cleaner" without em.

If you don't mind how they look, they only weigh a few ounces and will not adversly affect the operation of your bike.

L8R