Conceptually, I see this as seeing a list as its own thing, independent of its contents. That is, a List<Int> is a value in itself, that can exist even if no instance of an Int exists anywhere in the program; it'll just be a List of Int that happens to have zero Ints in it.

Probably makes more sense when you come from imperative programming background, where List<Int> is a piece of mutable state that you can construct and then fill, in two explicitly separate steps (and then possibly empty it again in yet another step). Then again, I believe even mathematicians are fine with ideas of an empty set, or of a one-element set being distinct from the element itself.