Since most of Amazon's negiotated rates with carriers are lower than what carriers offer to consumers directly, customers get lower prices and would benefit from Amazon's scale and customer service.
Now with Amazon's own transportation network finally taking off, Transportation By Amazon has gone from a concept that would have relied exclusively on third-party carriers (e.g. UPS, FedEx, USPS, etc.) to a likely reality with primary reliance on Amazon's infrastructure and supported by third-party carriers where Amazon does not have a robust prescence.