1) Better experience. I don't get this either, but for some people it's a big deal
2) Apple has A LOT more phones than any of their carriers. The carriers don't even know when and how much stock they'll get. Basically Apple jsut sends them some amount of iPhones whenever Apple feels like it.
3) Apple is known for sometimes bringing things out to the people waiting in line. In the past there's been ice cream at some stores, etc. Guess this is a minor deal, but does enhance point 1, I guess.
4) For this phone it will be perhaps easier to get apple care+
5) easier to buy accessories (more selection than a carrier store will have)
6) Apple employees are more used to this, and the stores are better set up, as they have multiple 'wait in line' releases every year. Even though the volume is substantially greater at Apple Stores they deal with the traffic WAY better than any of the carrier stores.