Basically it's cause you already know the pairs which you form and those are the same all the time as well.
In general it also means that you know what the S2L will be each time so you can always look for the same pieces instead of trying to work out what the pieces are later. If you are fully CN on megaminx, it would not really have as much of an advantage as on a 3x3 because the distribution of the pieces would usually be greater and more sides would have to be drilled with even more practise time than for 3x3.