Oh, I get to answer a quesiton finally! The reason is that razors periodically need honing, and many are not setup or not able to hone their own so they send it out and pay to have it done. Let's say that a razor in continual use would go 2 months between honings (this is for illustrative purposes guys, I've been using mine over two months and starting to feel like it needs to be honed, I'm sure it is dependent on many factors, steel hardness, type of beard, stropping technique).

now, with a 7 day set you are only using a razor every 7th day, so you could go 14 months between honings, theoretically.

I think most practically is a 2 razor set, because there are times I'm travelling and I think that if you were going to be on the road away from home and your usual resources having a backup razor in case something happened to the other would be great.

Red