A pressure reducing valve (PRV) is a valve that will take the supplied water pressure from the home and drop the pressure down to a lower setting. This is done to prevent the boiler system from being over pressurized whle it is being filled with fresh water.
 
In a system, the PRV is usally located right after a shut off valve coming from the cold water supply to the home. The reason for this shut off valve being before the PRV is so that after the system is filled, and at a proper pressure, you can turn off the cold water supply to the boiler. That's right  After the system is filled, turn off the cold water supply to the boiler. The reason is that a PRV was made for one main purpose, to be a labor saving device, not a automatic water fill valve for a boiler system.

Boiler systems have been around alot longer than PRV valves have been. The old way a boiler system was filled was simply throttling the cold water supply to the boiler system. This required two men to preform the boiler fill up. One man would be in the basment throttling the cold water valve, being very careful not to exceded the MAWP. The other man was up at the highest point, bleeding the air out of the highest radiators. If the pressure were to get to be too high in a system, the chance of actually causing a leak was greatly increased. This is why the man who would be in charge of throttling the cold water valve would be constantly watching the pressure gauge on the boiler, not allowing it to exceed 12 PSI in a single family home. The man on the top floors, bleeding the radiators, would be counting on the man in the basment to be providing enough pressure in the system, to force the air out, but not exceed the (maximum allowable working pressure) MAWP. As labor became more and more expensive the need was to develop a device that would eliminate the man in the basment. The device needed to be able to throttle the cold water valve, and also be very accurate at delivering a set water pressure to the boiler system. This device was called a pressure reducing valve.  

This brings us back to why the cold water valve in front of the PRV should be in the closed postion while the boiler system is filled and in operation. There are a few reasons. If the PRV were to ever fail, the water would leak out of the relief valve causing a huge mess, because nothing would shut the boiler supply water off. One of the most important reasons is that in the event of a slow, undetectable leak in a boiler system, the PRV would act as a auto fill valve. There would always be fresh water entering into the system. This not a good thing for a boiler system for two major reasons. It decreases the efficiency of the boiler drastically, and also decreases the life of the boiler. All manufacturers of PRVs instruct the operator to turn off the cold water supply after the system is filled.