If cases of equal capacites and bores are compared--
In big calibers we can get more energy out of a long straight belted case, as compared to short, shouldered one
of the same bore and the SAME CAPACITY; BY USING FASTER POWDERS. The main reason shoulder cases were developed was to shorten actions, save weight and still get high velocity,
compared to the older cases they replaced, and feeding
seemed to be better for military use in bolt actions and
machineguns.Ed.

To hold up their "jackets". NUYK, NUYK, NUYK.

And what about this: to avoid that it falls in the barrel?