I agree that is a very odd thing to find. It makes no sense to plug off a radiator hose like that. Why not just cap off at the radiator if you don’t need the flow??? And Kubota has one continuous hose between points on some M8200 models! And on some models that hose connects to the engine while on others it connects to an oil cooler (in the filter base).
But I don’t know what the part looks like! But the name of the part is “joint - drain” so that implies that it is used to drain the radiator and the block. See below. I believe you have that model tractor but you need to verify that is the correct model diagram. Please research all the listing for M8200 models and see which is the correct one for your tractor. The diagram below is the DT CAB version (I think).