Strange valve leaking

Hello,
I have a hot water system in a house that is relatively new to me. My previous house had a steam system that I got to know and understand fairly well. In this system there are 4 zones and each has a mechanical zone valve. Last year I had to replace the head on one of the zone valves. This year there is a corrosion leak on a no handle valve seen on the photo below. I don't know what this valve is used for and do I have to replace the valve or can I take it out all together. When I replace it or remove it I am sure I will lose water in the system. How do I add water back to the system or will that happen automatically?

Thanks so much in advance.
Regards,
Vinny