The reason for rotor rpm is to make the blades have enough airspeed, obviously. The max rpm is selected to make the blade tips (the fastest part) move at about 700 feet per second. This allows them to make lots of lift in a hover, but also avoid transonic speeds at cruise. Most rotors spin so that the blade tips hover at 675 to 725 feet per second.
This is why the rpm is higher for smaller radius helicopters - with less radius, it takes more rpm to make the needed tip speed.