I can’t speak for aurelien of course.
But the middle grey slider basically does what the exposure module does.
Raising exposure (with filmic enabled ) has the same effect has the middle grey slider.
But… Because now you have set the proper exposure before filmic, other modules now also know what the correct exposure / middle grey is (well, I’m not sure if the modules ‘know’ but the data has better values for them ) .
This means things like the tone equaliser, color correction RGB, color calibration, etc… (which all slot in between exposure and filmic ) have a better understanding of what are shadows and what are highlights (and what is middle grey of course ).
By using the middle grey slider in filmic , you keep that information only to filmic …