Unused fuel has to go somewhere, you can either send it back to the tank or back to the injection pump.
Since the fuel pressure on the injector return side is low, it is easier to send it back to the tank. Some tractors with rotary pumps also have a high pressure return which is sent back to the pump inlet, usually through the secondary filter. The fuel is also clean of contaminants and water, so recirculation keeps the fuel cleaner and moves the water to the bowl. The rate of recirculation is not very high, but if you have an electric pump with a constant flow rate it is higher.
The path it takes also allows refilling of the cold start system with fresh fuel, since it T's off the return line. This is the case for most brands and models that use a Ford style cold start system.