My guess would be for washing something -- maybe some kind of crop as you use it annually?
Looks as if you can step easily down to stand by the trough and tie whatever you are washing to one of the booms over. I would hazard that the circular structure is a filter to collect whatever you wash off and prevent it from going down the stream.
ETA - OR similar method to above but for purging carp?
It is for washing, but for rugs and blankets! The spout is offset so that it sends the washing round and round, like a modern washing machine. You throw in half a dozen rugs and some soap, then after 30 mins or so, let out the water and let it rinse. Often used for the great spring clean!
So you just turn up throw your rugs in!! I take it then rugs are used widely in Bulgaria then. We wash ours by power washing draped over the fence, though the neighbours not to keen amazing piece of kit. You are really selling Bulgaria to us