Final answer:
Constructed water reservoirs primarily store large amounts of water for use during dry spells, but also offer irrigation, urban supply, hydroelectric power, and flood control benefits.
Step-by-step explanation:
The primary purpose of constructing water reservoirs is to store a large volume of water for long periods of time. Reservoirs help to collect water during periods of excess rainfall and provide a stable supply during dry spells. Other major benefits of reservoirs include supporting irrigation of agricultural lands, providing urban water supplies, generating hydroelectric power, and helping with flood control. While reservoirs offer these diverse benefits, they also pose challenges such as environmental impacts, downstream river erosion, and potential dangers if not maintained properly.