They also make the arrow leave the string at the same spot every time and aids in speed from what I've read by having the arrow leave the string when the energy in the string is at its greatest instead of being released when energy is decreasing because the string has passed its centerline. If that makes sense.
Yes, you need one. My bow likes about a credit card-width between the string and the stopper. You can make your own, too - just get an extra cable rod, cut to length, and put a rubber stopper on the end...
I had a guy at a bowshop tell me if you use a string suppressor, you must also use a stabilizer. He claims if you don't have a stabilizer then the vibration from the string hitting the suppressor goes through your riser and causes vibration. Im sure most already use a stabilizer, but I never did up until late. That was just his advice, makes sense to me though.