What is the reason of having an Angle of Incidence on an airplane?
Strictly speaking, there is no need to set a particular angle of incidence- the wing will decide that for itself depending on the conditions (speed, weight, altitude etc.). What we are deciding is the The mounting angle, which is set for various reasons and is not variable in flight (except for some rare cases) including:


Mostly this is set so that the fuselage is (nearly) horizontal during cruise. This is especially important for airliners- DC-10, especially flew at a pronounced nose high attitude requiring the cabin crew to walk uphill.
Setting the wing at an incidence helps improve the visibility- this is important especially for carrier aircraft, where the pilot requires good visibility and also higher angle of attack (for keeping t/o and approach speeds low).
Setting the wing at an angle helps in keeping the drag low for the given lift. Having the wings at an angle and fuselage horizontal means that the drag is minimized, while the wing has the required angle of attack.


Though the angle of incidence is usually fixed, it can be varied in response to specific requirements. A good example is the Vought F-8 Crusader, which allowed the wings to be pivoted 7° out of the fuselage on takeoff and landing, resulting in a high angle of attack -- reducing the approach and take-off speed -- while keeping the fuselage level and giving the pilot a good forward field of view.



F-8 Crusader with the wing pivoted up during landing. By USN - U.S. DefenseImagery photo VIRIN: DN-SC-88-06695, Public Domain, Link

Another example is the Martin XB-51 which had a variable incidence wing to reduce the takeoff run.