In this topology, the Hub router will receive advertisements from R2 Spoke router on its tunnel interface. The problem here is that it also has a connection with R3 Spoke on that same tunnel interface. If we don't disable split-horizon, then the Hub will not relay routes from R2 to R3 and the other way around. That is because it received those routes on the same interface tunnel and therefore it cannot advertise back out that same interface (split-horizon rule). Therefore we must disable splithorizon on the Hub router to make sure the Spokes know about each other.