If you use a tiling WM, the immediate question which comes up is : can I switch this to a clean single-desk view, or is this neccessarily a tiled plane of n things, and this is one of n?

Tabs, are an attempt not to have to switch desktop, or tile the plane. Its a "stack" metaphor, with the least cost exposure of the underlying stack, as a shuffled hand of cards fanned out.

Hand of cards, vs one piece of paper per desktop but many desktops, vs I cover the one desk, but all things have to balance the space.