Require is probably not the word, it's more of a "why not" kind of thing. I have CPU, RAM and GPU blocks, with 4 radiators, two Koolance QD3T pairs, and probably something like 5 or 6 meters of tubing in my loop since my radiators are mounted outside my case. With all 3 pumps at 100%, flow rate is only 243 liters per hour. Plenty of flow, if I wanted to tolerate the awful pump noise. At 50% pump speed, flow is around 180 L/h, but still kind of noisy and it's starting to approach flow rates I'm not happy with. At 0% pump speed, my system is totally silent but flow rate is only 112 L/h and that's why I'll be adding 2 more pumps. Hoping they bring me closer to that 180 L/h rate at 0% PWM, or 300+ L/h at 100% just for fun benchmark runs. I have a Ryzen 9950x3D and RTX 5090 so the coolant can get pretty hot intra-loop, and higher flow rates help keep all components getting the same colder temperature water, not to mention help ever so slightly reduce component to water deltas.