Some backstory is needed to understand the belt. Most of the info is in the loading manuals but its usually given in bits and pieces under the individual cartridges. Here's a summary,
In 1912 Holland & Holland introduced the .375 H&H Magnum. It is a long, tapered case with a fairly small shoulder. They added the belt for reliable headspacing in bolt guns and double rifles. H&H also made a rimmed version (Flanged in British terms) without a belt specifically for use in double rifles.
Along with the belt, H&H also introduced to shooting the term "Magnum", which was taken from the wine industry, and indicated a larger that standard size bottle.
In 1925 they introduced the .300 H&H which is even more tapered with a long shallow sloping shoulder. Based off the .375 case it also had a belt, for the same reasons.
The shooting public came to very strongly identify the belted case with high end performance, and virtually demanded it for decades, even though the belt itself had nothing to do with cartridge performance, it was the acceptable style and people passed up non-belted cases for belted ones.
The magnum rifle rounds from Winchester, Remington and a few others, designed in the 50s 60s, 70s all used the H&H headsize and belt. Its what the buying public wanted and expected in those days.
The magnum rifle rounds of the next generation (ultra mag, and the various short magnums) were based of a different, non belted case, not the .375 H&H, and so, don't have the belt.
The belt essentially acts like a rim, for headspacing purposes, without the issue of rimlock a larger projecting rim can cause.
Its not the best system possible for extended case life, but it works, and is totally adequate for the non reloader.
Many reloaders adjust their sizing so the belted magnum case headspaces on the case shoulder, and not the belt. Done right, this extends case life, but it is the nature of bottle neck cases to eventually get thin just in front of the solid case head, and eventually crack there. They don't last forever.