This is used extensively for aircraft engine parts, like gears and driveshafts. Not all of their customers appreciate videos of their parts being "out in the wild", so a generic demo is used instead for videos. Let us instead say that these processes are being used every day for real parts, and have been at least since the 80's when I saw it.
As far as the two flat plates one, that can actually be a very good finished part, just like that, if the two halves are dissimilar metals. It allows you to weld using regular processes appropriate for each metal onto two structures that are dissimilar, like a steel hull and an aluminum cabin on a boat, at regularly spaced locations. It functions sort of as a mounting bracket without having to somehow get both large parts moving and pressed together.