I think it's really to get people to order more from Amazon, even for orders <$25. Tote will probably cost them enough that those orders don't make them anything, but it will build loyalty (and they let you keep the tote bag--keeps Amazon in your mind).
I'm not sure their is a benefit for the customer other than hoping Amazon will pass the savings on in the long run. However, this service might some what eat into the revenues of Amazon Prime.
Edit: I have had 3 Amazon packages arrive in one day - one by USPS, one by FedEx and one by UPS. I get mine at work, so I'm not bothered, but I can imagine it is a convenience for people who have to answer their own door for packages to know that everything will arrive in one go.