I'd say the reason is mostly for comedic purposes.
The default cube is often ridiculed for being a peculiar choice for default startup scene, and a lot of tutorials start by legitimately removing it, but a lot of them also end up needing it again anyway.
The punchline being you could easily have already saved multiple startup templates with your choice of default scenes, as easily as you could have delete it off-screen ahead of time, otherwise if it really is needed you could have just used the cube that was already there anyway; but people seem to keep removing it only to add it back again.
I suppose at this point it became sort of an in joke, emphasizing that the cube may not be that useless after all.
Objectively I don't think there is any measurable advantage in replacing the default cube, other than what Harry McKenzie pointed out which I figure would be irrelevant in most situations.