.. and, following on from MFS ..
For certification exercises, one needs a boundary condition ("rule") against which to measure compliance.
Many of the "rules" originated in "finger-in-the-wind" professional assessments by the chaps who ran the show in the early days of commercial aviation.
So, for example,
(a) the initial screen height of 50 ft dates back to a demonstration of an early aircraft to the US military. Apparently, the parade ground from which the demonstration flights were conducted was surrounded by a tree line of such approximate height ... seemed a good idea so the FAA ancestor adopted it.
(b) the 70 mph stall speed limit (dutifully carried across to the modern era as 60.8 kt - rounded to 61 kt - in FAR 23.49(c) ) originated in an early engineer's need to have a figure .. consideration of motor vehicles apparently led to the 70 mph figure's being thought to be a reasonable place to start from ....
Vmcg, being an extreme certification boundary, is not often encountered in routine operations, but we need a "rule" against which to show compliance .. the accident record's having shown the definition to be reasonable, it hasn't altered materially.