Drain for a sink would not get a valve. Not unless it had constant flow and could not be interrupted and required servicing. However many drain lines, from such applications as storage tanks and whatnot will. All depends on the drain. In an aquarium, such as mine, I have dual overflows and one side is considerably louder than the other. With valves on both drain lines it allows me to throttle down the louder side to quiet it and still get flow. Furthermore, I had an anemone decide it wanted to campout on the side of an overflow box. Started getting sucked in & I couldn't remove it without damaging it's foot. With valves installed, I was able to stop flow on that side and allow the anemone to right itself and then move (due to change in current), all while still minting flow through a sump. Sorry crister if thread got jacked.