If this is a two pipe system, the cold water side brings the heated water back to the water heater. The bridge valve. Is a check valve, water flows in one direction and prevents cold water mixing with the hot water side when water is drawn at a faucet. It is essentially a hydronic heating system . As the pump runs heat get dissipated from the pipes.
it's why the timer can run in intervals. If you set it for 24/7, your fuel or electric bill will go shy high since your heating the home at all the time. Another is your hot water may be ok when first opening the faucet, but if you go cold, the heated water will flow from the cold pipe until the cold water flushes out the heated water. It's all a balancing act. Timer on such as mornings and evenings. Occasionally during the day if someone is home all the time.