To add what I recall from an article about the .30-06 / M1 Ball / M2 Ball, the '06 was originally loaded with the 150-gr flat based bullet. Doctrine evolved for long-range firing at troop columns, convoys, etc., sometimes even out of sight of the firing infantry units. The M1 Ball with its 172-gr boat tail bullet was developed to extend the range of plunging fire at area targets. With the development of the mortar, long-range fire at area targets by infantry units was now possible with man-portable artillery, so the long range of the M1 Ball was no longer necessary. Since it recoiled considerably harder than the .30-06, (and possibly due to range issues as previously mentioned) the military adopted the M2 Ball cartridge, which was, essentially, identical to the original '06 in ballistics.
As has been pointed out, with millions of M1 Ball rounds in the inventory at the beginning of WWII, it would only make sense to load them into machine gun belts where the excess recoil would not be a factor, and where extra range and penetration would be desirable.