For map makers how make custom resource pack for their maps, they might have to make more than one resource pack if they want lots of custom sounds. Also so people don't have to manually change to a resource pack when starting a map they can press a button and have a command block do it for them. This adds a whole new possibility for map makers. Also in the pack.mcmeta file for the resource packs have a name in it to type after the command to change the resource pack
It would still be cool if you had stages to your map and needed more textures for each part
Meh... there are enough blocks to do that already... I think if you need like 3 different resource packs, you're(I used the proper "you're" cuz imma gangsta) going a little overboard with the whole custom textures thing.