Much as I am not interested in either smart phones or tablets personally, the reality is that lots of people are
interested in them and do editing on them -- often to a significant level -- before posting stuff to the Internet, or saving it off to their silly Cloud thing (I wouldn't touch that with a 40 ft barge pole, personally), or saving it for transfer later to their main computer, or all three of the above and possibly some more as well.
I reflect that my workhorse computer is a two year old laptop. Five years ago, I was just
contemplating switching from a large desktop tower. A couple of years before that, the notion of having a laptop as my only computer was laughable. Laptops just didn't have the computing power.
Today's smart phones and tablets are vastly more powerful than early laptops and so far ahead of the desktops I first did DTP and later edited photos on that any comparison is laughable.
In addition it is just plain good marketing to produce an app. More and more people are shifting work or starting work on their phone or tablet, and for many, that is the entrée to computing. When they move up to a lap top (or bizarrely, a desktop) obviously they will go with the name they know. iPhone and iPad users are buying Macs, and they are installing on those Macs the programs they are familiar with in the form of phone and pad apps.
The problem with the projection regarding producing stuff using a smart phone or tablet t Starbucks is not the phone or tablet, it is Starbucks. It serves standard American coffee, and that is not coffee, it is crap! As a result, Starbucks is contracting (including, thankfully, here in Australia where we take our coffee seriously) while Apple is expanding, and has now become the richest corporation in the world.
And lots of developers are doing very nicely surfing along in Apple's wake, producing apps.