You should think of it really being as Hitman season 2. So there is now a new program that contains all the modern Hitman stuff within. There's no need to use Hitman 1 any more. Even if you don't own Hitman 2 but you do own Hitman 1, you can download Hitman 2 and play the old levels inside there.
What benefit does this give? I have HITMAN GOTY installed and I don't plan on buying HITMAN 2 until it becomes cheap, so what would I gain by deleting HITMAN and downloading HITMAN 2?
EDIT: Just noticed this is consoles and not PC -_-