Why do you still drain urine out of a foley catheter when you also have bilateral nephrostomies?

Nephrostomies: It depends on the reason for the bilateral nephrostomies. Often there is something obstructing the forward flow (often the prostate, bladder mass or stones) and the Foley catheter can bypass it enough to drain some urine.

Nephrostomy: Urine will always drain with gravity to the path of least resistance. If you have bilateral nephrostomy tubes and patent ureters, all four paths will have equally minimal resistance, and urine will go wherever gravity leads it.