The requirement for the fuel return line is specified by Rotax.
My understanding is that the carb. floats are very sensitive to excess fuel pressure. Any pressure beyond what they are rated for, and fuel will leak past the needle/seat, drain out of the carb and wash oil out of the air filters, etc.
This doesn't occur in flight with a properly designed/installed fuel system, but it can occur while parked when the fuel system gets heat soaked. The check valve in the engine driven fuel pump prevents any back flow so the expansion of the heated fuel will start to raise the pressure in the portion of the system that is after the engine driven fuel pump, and then the carbs start leaking.
The fuel return line connects to a fitting on top of the engine that has a very small orifice in it. It has a rather low fuel flow rate at normal pressure but provides a pressure bleed off point when the engine is stopped... to prevent the problem described above.
Since the normal operating pressure is relatively constant, the small fuel flow of the return line can be compensated for with the K factor value in the fuel computer