Automotive carbs usually only operate a one altitude most of the time, thus they can get away with a fixed mixture setting supplied by the jets and or metering rods.
Airplane carbs have to be able to operate over a wide range of altitudes, from sea level, all the way to 15000 feet or even higher. So, a way of adjusting the mixture is required for optimum profomance. At higher altitudes, leaning for best power is mandatory for satifactory perfomance.
The mixture controll on aircraft carbs is essentially a movable metering rod.
As far as the Caravan and all those power levers... what would be the mixture lever in a piston engine is the "condition" lever for a PT6... it essentially turns the fuel on and off. The throttle lever on a turbine is the "power" lever, and it adjust the fuel metering to the power section. The more fuel and air, the more power a turbine engine will make, until you either reach a temperature limit or a power limit set by the manufacturer. There is no lean or rich mixture, a tubine engine needs as much fuel as there is air to burn it with. That's why higher is better. The less air there is, the lower the fuel burn.
Keep in mind that a gas turbine is a completely different animal than an internal combustion piston engine.
There was a really good set of article in AOPA pilot a few years ago about the fuel metering circuit and power outputs in turbine engines. If I can find them, I'll post them here.
The "beta" range, or reverse, is on the power levers as a mater of lowering pilot work load. The didn't always used to be there. I've seen older aircaft with "reverse" pitch toggle switches.