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This picture is of a Watts 9D backflow preventer it is installed on the feed line to a hot water boiler, it's required by code on all hot water boilers. it's purpose is to prevent the dirty, toxic water from the boiler from getting into the domestic water system (the water you use to cook bath and drink)  One side of the copper pipe connects to the boiler fill valve that sends water to the boiler, the other pipe is  connected to your drinking water. The opening in the middle facing down is the vent or air gap that will stop a vacuum that would allow the boiler water from coming into your drinking water. this fitting needs to be open to allow air into it if a vacuum happens.
Some knuckle head saw that the vent was dripping, so he or she installed a boiler drain in the vent opening, (instead of fixing the Watts 9d Backflow preventer) and it looks like the boiler drain also leaked so a cap was installed on the threads of the boiler drain. This is equal to removing the backflow preventer all together. A real licensed plumbing contractor would have never made such a foolish mistake. Some boilers contain antifreeze, antifreeze in your drinking water could kill you.

Watts 9D back flow preventer