For this discussion I will use the "Pacific Fighters" terminology of 100% is "fine pitch" and 0% is "course pitch". In "Pacific Fighters" fine pitch is 100% pitch setting (default until adjusted), while coarse pitch is 0% pitch setting and all the values in between are the adjustment from fine to coarse.
A fine pitch propeller has a low blade angle (more "vertical") and will take a 'small' bite of the air, each rotation will try to move forward a small distance through the air. It requires relatively low power to rotate, allowing high propeller speed to be quickly developed, but achieving only limited airspeed. This is like having a low gear in your car.
A coarse pitch propeller has a high blade angle (more "horizontal"), will try to advance a long distance through the air with each rotation, it will take a big 'bite' of the air. It requires greater power to rotate, limiting the propeller speed that can be developed, but achieving high airspeeds. This is like having a high gear in your car.
Fixed pitch propellers are very limiting as they must trade off speed/power vs economy. A propeller designed for economy would be unable to develop high speed.
If we have a variable pitch propeller, why would we use a fine pitch for fastest speed? We just stated a fine pitch propeller has limited airspeed. True, however it's the matching of a propeller (and it's pitch) to an engine's abilities (RPMs & power band) that achieves the best speed. If we were to use a courser pitch to gain more speed we would load the engine, reducing RPMs and drop below the "power band" of that engine thus resulting in less power and less overall speed. For fuel economy however we desire lower RPMs and thus we adjust to a courser pitch for cruise or economy flight.
Constant speed propellers (a form of variable pitch propeller) have an RPM adjustment lever and will maintain (via automatically adjusting the prop pitch) the set RPMs independent of other conditions. Higher RPMs draw more fuel and create more drag. An aircraft has only one or the other, either an RPM adjustment lever or a prop pitch lever.
When decreasing power, decrease the throttle first and then the prop pitch. When adding power, advance the prop pitch first and then the throttle. - this is very important (but apparently not modeled in "Pacific Fighters").
When landing prop pitch should be 90% to 100% (more drag and the ability to increase thrust if a go around is necessary). Alternatively you could land with zero pitch (no drag) for a nice unpowered glide in and then use pitch control to create braking (with the throttle off). 100% is fine pitch or full power (like 1st gear in a car), 0% is course pitch (like 5th gear in a car) for economical cruise.
Fine pitch allows the engine to reach maximum speed and hence maximum power at low air speeds. Vital for take-off, climb, and for a go-around on landing.
Coarse pitch allows the desired aircraft speed to be maintained with a lower throttle setting and slower propeller speed (lower RPMs), so maintaining efficiency and improving range. Thus a coarse pitch allows improved fuel efficiency and greater range.
Coarse pitch will ensure your engine does not over rev while the propeller absorbs high power, producing a higher top speed. (useful for those steep dives)
With a fine pitch and low throttle setting, a slow turning propeller is able to add to the aircraft's drag, so slowing the aircraft quicker on landing.
As a general rule keep your prop pitch (rpm) setting at or above your throttle setting for power. For economy drop your prop pitch below your throttle setting. (this is specifically directed to flying in "Pacific Fighters")
Fixed Pitch Props: TB-3 and P.11c no pitch control!
Variable Props: Fw190 & P.40 & Bf109E
Constant Speed Props: These are the most common, with these you control the RPM not the prop pitch.
Automatic Control: Fw190D; Bf109F & Bf109G & Bf109K normally automatic but can be switched to manual variable control. Fw190A & Fw190F automatic only!