A nice thing about physical versions is that physical copies that go on sale in the used market or get marked down for sales in big box stores and Amazon is that their existence puts pressure on the digital releases to keep their prices in check. Why should I buy digital when the physical version might/will go on sale for $10-$20+ cheaper?
That's one of the things I fear about an all-digital marketplace. With no physical releases out there, I think we'll see sales on AAA digital titles a lot less often.
RE: Why?