Whenever you disable mob spawning using /gamerule doMobSpawning, it stops the spawning of ALL mobs. This can be useful for prohibiting natural spawning. Lets say you wanted a world where you still grew hungry, but wanted to keep AGGRESSIVE mobs from spawning and let PASSIVE mobs spawn naturally. You could simply add the type argument, like this: /gamerule doMobspawning agressive false and now you have hunger without monsters and with animals. This would be very useful for maps and general gameplay, as it would allow you to play with the fun of finding food without being attacked. You could have a cafe or restaurant in your Peaceful mode city that served food your friends would really need, among other uses. To prohibit animals from spawning, you would say /gamerule doMobspawning passive false. To prohibit all mobs from spawning, you would say /gamerule doMobspawning all false. This would be a simple way to make that possible.