Asymco (I wish they attributed by real names so I could reference that):

If the current download rate is maintained (17 million apps/day) and if the pricing of $0.29/app is preserved, then $1.8 billion will have been spent on iOS apps this year.

If you want to know the precise reason everybody and their mom wants to have their own app store, take a look at the graph they posted. Insane growth.