A rubber stamp impression. Two flat colours, ink and paper, and nothing in between.
The picture is blurred, its brightness is cut at a threshold, and the two sides become the two colours. The blur is doing the real work here. Cut a sharp picture at a threshold and the boundary follows every fleck of texture, giving a ragged noisy mess. Blur it first and the boundary follows only the big shapes, ignoring detail entirely, which is exactly what a stamp does when it reduces a picture to a single cut shape.
So smoothness is really a control over how coarse the shapes are that survive the cut, not how soft the edges look.
A separate roughness control wobbles the threshold slightly, giving the edge the torn, uneven quality of ink pressed off real rubber rather than a mathematically perfect line.