Same reasons here. Some redundancy for flight controls if one receiver fails, as splitting the load is another. With the reliability of the receivers these days, is almost questionable whether or not the redundancy aspect is worth it. However, at $6-10K per plane, the extra $200 is real cheap insurance for me.
Both JR and Futaba only recommend connecting the number of servos to match the number of channels of the receiver. If you have a 9 channel receiver and 14 servos, its best to use two receivers. At least that is what the people that designed and are selling the receivers say, so I go with the flow
