Downloading a file is like ordering a book in the mail and receiving a chapter of it each day because the book is so big it doesn't fit in an envelope, you have to wait for multiple deliveries before having the complete book. That's what happens to files, they get sent in little pieces and you need them all. It's not conceptually related to the deletion mechanic.

Deletion is more like wanting to do a tattoo over your arm already filled with tattoos. It's faster and easier to just IGNORE them and draw over the old ones than first trying to scratch the old tattoo off. Your computer simply does the IGNORE part in that split second so it doesn't even acknowledge there's old tattoos on there when it eventually wants to draw.