This is an expansion tank installed on the water supply pipe to the water heater. Its purpose is to handle the thermal expansion of water as it heats up in the water heater - to prevent water pressure from getting too high. If water pressure gets high enough it can damage valves in plumbing fixtures, joints in supply pipes and even the water heater. However, it is not recommended to install an expansion tank sideways (horizontal) without a support strap. If the expansion tank is not properly supported and it fills up with water, the weight of the water might damage or even break the coupling. It is better to install an expansion tank vertically but if it installed horizontally, a support strap (1" wide) should be installed.
