I'm looking to do some DIY plumbing work, but this fixture confounds me. It looks like the biggest water hammer arrester, I've ever seen; but it can't be that simple. What is it, and what will I lose if I remove it?
I've never seen one of these (old ones) myself, but following Ed Beal's comment on a different question it's probably "an early form of damper as it was full of air" which could be as "large 2" pipe tee after the main valve with a stubbed leg verticle for 4-6 feet". A bit of googling can find similarly tall (and old looking) ones, although perhaps not quite as tall as yours (n.b., the one below was labeled "primitive"):
I had a house once that coulda used this. the water company or the fire Dept flushed hydrants and someone closed one too quickly. the resulting hammer continued for an hour and was audible in the street. we had problems with several stop valves afterwards. a whole- house hammer arrestor is worth the trouble if your water company or city building official suggests arrestors.