[formerly_proven]

2.) is there for individual components (prefix reference with #), but not for schematics. It would be pretty neat though if you could use hierarchical sheets for whole-system design that way, though. Note that you can have multiple PCBs in one PCB file. Kicad doesn't really care.

[m0zg]

It's less useful for individual components - I can already represent those with pads without affecting readability of the schematic too much. But I sometimes would like to carve out a chunk of stuff onto a separate board, and connect it with, say, a ribbon cable, yet keep it on the same schematic sheet, or at least within the same project. That is not doable in KiCAD 5 at least.