Since the launch of the original iPhone back in June 2007, the only place you could (legally) get an iPhone in the US was either at one of Apple’s 197 retail stores or one of AT&T’s 1,800+ retail stores.
As of Sunday, that number of locations has increased by almost 50% as close to 1,000 Best Buy locations are now selling the iPhone 3G. This makes them the first third-party retailer to be allowed to carry the iPhone.
The main benefit (at least to Apple) is that this obviously expands their retail saturation giving them the ability to snag the casual shopper who might not normally be shopping in an Apple store. There is, however, no economic benefit to the buyer by purchasing from Best Buy as prices for the iPhone 3G will be exactly the same as they are at all Apple and AT&T locations.