Hello Ian
If you don’t configure subinterfaces on the router, then no tagged traffic will enter the interface. The switch is set up to send frames to the router using dot1q encapsulation. This is where the VLAN number of each specific frame is added to the header as a tag. When the router receives these frames, it will drop them because they include a tag. By adding subinterfaces and the appropriate dot1q encapsulation, you are allowing the router to be able to receive tagged frames and to allow them to egress on the appropriate subinterface.
If communication between two devices on the switch that are on the same VLAN occurred, those frames would never reach the router. This is because no routing is necessary since both devices are on the same subnet. So, same-VLAN communication would not be effected.
I hope this has been helpful!
Laz