the way i had it explained to me was it is like cruise controll on a car, the govener will hold the head speed on a heli constant so there is no need to set up throttle curves it just makes it easyier.

say if you are doing a seiries of 3d tricks and the head speed starts to bog, with throttle curves they wont increase the throttle anymore, with a govenour it will increase untill the motor hits its maximum power and when you come out of the trick it will then decrease the throttle so the head speed dosent go to quick.