How do i use the throttle vs rpm regulator in a baron 58? Why are there not just a throttle to control the speed?

Think of the RPM control like a "stick shift" in a car. If you just cruise along, you take a high gear and the engine will turn at lower RPM - that way it uses less gas. If you really want to accelerate or go up a steep hill, you need to shift down and make the engine rev high - so it can develop full power.

Same in an aircraft with "constant speed prop". You can set the RPM that the engine will try to turn at, so you have maximum power for takeoff (high RPM) and lower fuel consumption (lower RPM) during cruise flight.

The set RPM is achieved by the propeller automatically, it will turn its blades so they "bite off" more or less air, that way the engine can turn faster or slower.

Cheers, Jan