Conventional Wisdom says that long ago,when they first went from all brass casing to part paper, the high brass was to contain higher pressure loads and keep them from blowing out back towards the shooter.
In actuality,the lower pressure loads were also(surprise!!) lower price loads, and the lower brass was the indicator of same. Customers demanded high brass for more expensive loads due to that Conventional Wisdom,and it became a tradition.
Now, it serves as a nice way to ID heavy loads.