Draining Water Heater
To drain it, you need gravity working with you; you don't have water pressure because you've turned off the incoming cold water.

Your sink us attached to the top of the water heater, and in most cases is located even higher up. You can't drain that way unless you turn the house upside down.

So instead you open a hot water tap somewhere to let air into the system, then open the valve at the bottom of the tank to drain its contents. You'll presumably want a hose and/or buckets to move that drained water somewhere appropriate -- route it into a sump, perhaps.