What is the RPM? I know what RPM is, but not sure what you are controlling in the game.
You're changing how many rotations per minute your prop makes. If you look at your control panel, and see the RPM, you'll notice that it stays the same whether you are at full throttle or at idle. What you don't see is that although the prop is still spinning the same speed, the angle of deflection for the prop is different. It isn't taking as big of a "bite" out of the air, and/or producing as much thrust at a lower throttle-setting as it is when the throttle is maxed...
max throttle and max RPM is where you want to be while fighting (it's where you'll get the best speed and acceleration), but you're burning lots of fuel at that setting. Once you're level and "up to speed", you're not really gaining that much speed, and don't really need much acceleration, so for traveling back and forth between fights you may want to reduce throttle
and RPM to get better mileage. Reducing throttle helps, but not as much as reducing
both. I generally only reduce my settings while RTB low on fuel.
To see the "best" settings, just open your clipboard and click on E6B. Looking down, you'll see the max cruise, max power, etc, settings. You just adjust your RPM and throttle to those settings...
Another way to use RPM is to reduce RPM to a minimum on a multi-engined plane if one engine stops (damaged). A propeller spinning fast creates more drag than one spinning slow. Er, rather, the drag is what
makes the prop spin faster... Taking this a step further, say you find yourself in a fighter, RTB, and your engine quits (but the prop is still spinning). Reducing the RPM to its minimum setting will allow you to glide quite a bit further...