[jnaas2]

The valve you are talking about is more then likely a pressure valve. When water heats up it expands and if there ie a backflow preventer in the water meter it will cause the water pressure in the house to increase until it blows a line or causes the pop off valve on the water heater to release the excess pressure. So plumbers install a small expansion tank on the cold water side near the water heater or a pressure limiting device like you are talking about. If you cap it put a pressure gauge on you water line and see what the pressure climbs up to, I think you will be surprised. You are right in asking questions, NEVER CAP A LINE NEAR YOU WATER UNLESS YOU KNOW WHAT IT DOES. This can cause the heater to explode if the wrong line is capped and can kill