In an air brake system on a trailer-truck combination, what are the differences between a quick release valve and a relay valve?

They both seem to have the properties of speeding up the braking application. The quick release releases air faster as it has a exhaust port that’s located close to the brake chamber, whereas the relay valve has the property of releasing and applying air brakes faster, yet does not have an exhaust port (of my knowledge). Why have a quick release valve when you could just install relay valves and have best of both worlds— that is, fast brake application AND fast brake release....