Rotors are wing surfaces. You'd be surprise how much lift they produce in forward flight.
They are quite inefficient. The retreating blade is the limiting factor, with only around 40% of it producing useful lift, and at a high AoA with resultant drag.
The advancing blade has to throw away all that beautiful lift it gets from the added forward airspeed, to match the poor old retreating blade.
If both blades were advancing, lotsa lift both sides, no need to worry about retreating sides, hence the ABC test aircraft and Hokums and intermeshing tandems.