The belt was added initially, to headspace on, and belted cartridges still supposed to be headspaced on the belt. I would presume with the extreme taper of the 300H&H, and less so the 375, it helped. The extreme taper on the 300 case to help it be extracted as didn't have to move much to come lose. Never heard the 300wby fame was that it would "accept" 300Win mag cases, but since it's formed , initially, from a 300 H&H, should accept that one.
There is nothing either good or bad about belts. They were put on several British cartridges (i.e., 300 H&H and 375H&H) to control headspace in the era that the British used Cordite powder and long sloping shoulders that made headspace control difficult. With modern sharp shoulder cartridges, they are not really needed, but look cool.
