They simply don't have enough mass to protect your arm if that is what one thinks. They affect the sound the strings make at contact, and to some small part, the feel. How much that "feel" is or is not needed in your game is a personal thing. I use one because I am (prehaps too much) tuned into the sound the strings make. Simply, I prefer it "muted."
For anything beyond that, you'll need to do something like change the strings in some way, weight the racquet, or soften up the grip.
As someone noted above, they've become something of decoration. I'm sure players would use all kinds to suit their interests if more were available.
Still with the Head LM Radical MP/OS. The dying art of the 1HBH. Gamma X-2. Would a higher-end machine get me more forum "cred"? ;)