had to admit, never thought about lens swapping. I think i do understand the concept of it. changing lenses to change focal length in what is essentially a one exposure that is kinda a two exposures :)
could you do it with a zoom lens that has a decent range? for example 55-250.
Yes the main reason is to have different focal lenses. The steel wool on the ground is basically a macro shot or very close to it and the scene is a wide angle shot. So to get that in one I switched the lens mid exposure. And yes, a single exposure, just need to make sure it is very dark when switching the lens. Also some cameras don't like it if you take off the lens, but using analogue lenses usually circumvents that.
It does work with a toom too, did that already, but problem there is getting the focus right. You can use some markers on the dial but it happens quite often that you miss it a bit and some parts are out of focus. So switching lens and locking to focus ring with some tape is way easier.