Well, there's your answer.
It's used by a cooper in making wooden barrels. One end rests on the floor, the other is raised up to about waist height on a pair of legs. The bed of the plane is uppermost, with the cutting edge showing. The cooper pushes individual barrel staves down the slope, over the cutting edge, to form the bevels that will fit together to make a leak proof barrel. All done by eye, without any jigs.
This one is from the twentieth century, older ones were made of wood.