I don't know either, but if I had to guess, I'd say that iron thing is something called a "drum trap".
A long long time ago in a land far away, before they had P-traps, they had "drum traps".
The way a drum trap works is by having the incoming water enter the drum near the bottom, and the outgoing water exit the drum higher up. That way, the water level in the drum would always be at the bottom of the water outlet pipe. Tha would cover the inlet pipe with water, thereby preventing sewer gas from going up the inlet line and into your house via the drain lines. Solids would accumulate at the bottom of the drum trap, so there was a removable cap at the top to allow the drum to be cleaned by hand. Also, the above drawing shows a vent line connected to the drum trap, but it's not necessary to have one; most drum traps only had an inlet line below the outlet line without any vent pipe.
Most likely that open pipe to one side of the drum trap is an old inlet line that's no longer in use. If you take the cover off that thing, and find that the open pipe connects to the bottom of that thing, and there's another outgoing line about half way up, then it's an old drum trap that's no longer in use.
Also, drum traps were typically set in the floor so that they could be covered with a steel or plywood trap door (no pun intended) and be out of the way when you didn't need to access them.
Drum trap installation for a bathtub. You can see that the water remaining behind in the drum trap would prevent sewer gasses from coming back up the tub drain.