Minecraft wiki entry for Shelters, associated text: If playing with flying enabled (for example, in Creative Mode) you can create platforms at the same height on multiple buildings for easy flight between the upper levels of different locations. This works great in tree-forts, cities, and anywhere that multiple balconies are otherwise unconnected. Since 1.11.2, firework rockets have allowed the player to have a propelled flight with elytra, so these platforms should have water to slow the impact due to how fast players can fly, if these are meant for elytra.