Simple really, it is for normal and reverse.
Easiest to explain is your rudder.
You set up your aircraft and test it's functions, you move the rudder left on your tx but it moves right on your aircraft.
Now you switch your rudder channel to reverse and everything works as it should.
It depends on the aircraft and which way the servo is mounted as to whether you have to reverse it or not.
To take your heli pitch as an example. If you left it at normal and you leaned your stick forward, the heli would pitch backwards making it very difficult to control.
LOL ok Paul, you type faster than me.
