Wayland. It comes up a lot: “Bug X fixed in the Plasma Wayland session.” “The Plasma Wayland session has now gained support for feature Y.” And it’s in the news quite …
Because Pipewire only handles and understands media streams, so it can stream the output of a window or the whole desktop, but only because the Wayland compositor has already composed the windows and other data it gets from the application to a visual and hands the final result to Pipewire.
Which goes back to oroginal question. Why pipewire if there are already wayland extensions?
Because it is convenient for programs to use Pipewire for screensharing, as those programs can then also use the same Pipewire support for all their audio and webcam needs. Also Pipewire is good at multiplexing the various media streams.