Do you mean just setting the pitch to some value and using throttle to control altitude? There are a couple of good reasons for using collective pitch:

1. Using only throttle to control altitude will translate into relatively slow changes in lift (altitude) because it takes some time for the motor to adjust speed. When using collective pitch with a proper throttle curve, motors are already spun up to a high rate of speed, so changes in the pitch of the blades will translate into almost instantaneous changes in lift.

2. Generally, the faster the rotor turns, the more stable the heli will be due to gyroscopic effects. Using only throttle with fixed pitch necessitates that you spin up to a certain RPM to climb to hover altitude and then reduce RPM to hold the hover, meaning that the heli will become less stable hovering than when ascending. When descending, you would have to reduce RPM even further, making the heli even less stable.