Well if for example some merchant would accept bitcoins, that most people don't have access to, it would be nice to buy some btc first with your cellphone and then pay for the merchant. Other option might be to pay your friend with BTC that you can aquire instantly.

One of the biggest obstacles in BTC adoption is that it is too slow and difficult to obtain.

On the other hand, if two people have an iphone that can send funds to eachother, then who cares about BTC really... but what if the other phone would be android?