One reason: if the forger were unhappy with his results for some parts of the text he was forging, and had another go at those parts. In that case, those parts would end up in a separate layer. (I should add that I have significant experience in editing imagery for (legal) use on the web, and have practical familiarity with the vagaries of layer generation during such editing).
Also, in the case of the "Name of Hospital or Institution (If not in hospital or institution, give street address)" line I cite above, both the form box in which that text appears, and the box above it are typewriter-entry boxes, with the typewriter entries coming nowhere near the "Name of Hospital or Institution . . ." text. Even if the forger had placed that text on the same layer as elements of the document that were being forged, it seems implausable that any mistake would have affected that text.