Ron, pimmnz has covered much of it but I'd like to add a couple of thoughts.

With full size aircraft the incidence or rigging angle is very important. Real planes normally fly for long distances and they want the fuel economy to be as good as practical. So the designers set a cruise speed and "normal" weight. Knowing the speed and weight and wing area and airfoil test parametersthey can easily figure out what the angle of attack in flight will be. Contrary to pimmnz we can actually do that as well if we want. We can make an educated guess at the weight and pick a flying speed. From that we can figure out what the angle of attack will be to generate the lift needed. we would then set that as the incidence angle so that the fuselage is flying as directly into the oncoming air as possible to generate the least drag. Pimmnz, the tools are all avaiable online to do this. Normally I'd have agreed that it's not practical but other folks have wrapped up the equations in various online applications such as Xfoil and Foilsim to allow us to actually do such a thing.

Now that's all fine and dandy but it's not how we fly our models other than some very special cases such as thermal sailplanes used for contest work where they want to achieve the minmum drag during some speed runs or for best L/D at the best cruise speed. The rest of the time we only fly at a single speed for a very short time and distance. If we hit the speed where the fuselage is dead in line with the airflow then it's good luck more than good planning. Normally we're constantly climbing, diving or turning.

The reality of this becomes the fact that we modelers only need to use the incidence angles of the wing and stab to establish the basic decalage angle, or longitudinal dihedral if you wish, to set the wing to stab angle that will be used to achieve pitch stability. We don't really need to even do that if we don't mind having the elevator angled up so it's not flat to the stabilizer itself. But most of us don't like that look so we try to minimize such things by incorporating some angular difference between the wing and stabilizer to "get in the ballpark.