Many but not all patients on dialysis still make some urine. They will generally make urine once or more per day. Those that still have urine output have a better time with fluid retention between dialysis treatments. Since they make urine, they do not have to have as much restriction on their fluid intake. Any urine they make, allows them to take more liquid in the diet and not gain excess fluid weight. Once urine output ceases completely, then more strict fluid restriction becomes necessary in order to avoid fluid overload between dialysis sessions.
Dialysis patients who get ascites (fluid retention into the abdomen) would likely have problems with fluid retention and likely need more dialysis than they are currently receiving.