The original purpose of the belt was for head spacing. The first cartridges with belts (I think the 375 H&H was the second such.) had either no shoulder or as in the 375 H&H a shoulder that had too gentle a taper to provide good head spacing. In later years the belt came to be erroneously associated with more powerful cartridges. It then became an advertising gimmick to put a belt on your super duper 900 yard elk blaster even though the cartridge was likley to have enough taper in the shoulder to head space correctly on the shoulder. Today some 375 H&H rifles actaully head space on the shoulder although the correct method is to head space on the belt.

The best example of head spacing on the belt today is probably the 458 Winchester.

There is also the 450 Marlin with a rather wide belt, the purpose of which is to prevent chambering it in a 45-70.