Well, there are three reasons that I know of,

1. There is a technical argument that says you should let a razor rest between shaves. It something to do with letting the iron molecules settle or something. Incidentally, I have never noticed the difference between rested and non rested blades. My belief is if you don't use them regularly, preferably daily, the edge will start to corrode with rust. The longer you leave them, the more the natural oxidation process has to take the edge off your blade.
2. In times gone buy, the Gentleman would have his razors attended to by his butler. What better than a seven day set for the butler to take care of.
3. Why sell one when you can sell seven.

Take your pick but I think number 3. wins.