Honestly, just getting back to the point that you can write your own app and easily install it on your friends' phones would change the world. The amount of interesting stuff we lose to the inability of hobbyists to tinker with their technology is incalculable.
That doesn't allow you to do a large number of things that ordinary apps can do -- or that "approved" apps also can't do, e.g. by modifying parts of the system in advantageous ways.
You can also obviously just buy a PC and run whatever you want, but the entire point is the barrier to entry. If it's easy, more people do it, and that's good. If they have to contend with a convoluted mess, people give up, and you lose whatever they might have created.
