It's done for visibility. A retail presence helps a great deal for exposure.I mean...what's the point of the physical box if there's no cartridge? I'm not planning to get this for Switch or anything, but I don't see the point of the physical option for Switch then. Might as well put out a statement saying that the Switch version is digital only. I think that would be a better way to handle it than letting unsuspecting people buy a case, expecting to get a physical cartridge, and only get a code that they could've just stayed home and bought without wasting gas or time.
Edit: Or in worse case scenarios, get home and realize it's a digital code, and they have shitty or no Internet at all. Would it even be able to be returned then if it's a digital code?
And I'd have to assume there would be a notice on the box. They do it for when partial downloads are required.