Design considerations to manage water.
On old valve instruments, the water key is after the valves, where modern instruments have the water key before the valve section. In early instruments the water was a part of the proofing of the valves.
With old rotary valved instruments, you may observe that an instrument that was out of use for a long time will refuse to play at all, until you fill some water in the mouthpiece. Then they play, often quite well.