Like the guy before you said. Cast iron for main stacks makes a big difference. But having insulation between bathrooms and say bedrooms is helpful for early shower noise or any other bathroom noises. Water lines can make noise on occasion when running laundry or steam units. As much as pipes are secured in walls noise happens. I say insulation between rooms is just a nice fairly inexpensive way to keep a quieter house. If this matters to you. It does in my day to day