They might do it out of necessity: to "scratch itches".
Limitation breeds creativity. This is true in general, but certainly in computers. Ever heard of demoscene? By comparison to the constraints we had to work with in the 80's, the power of today's handheld computers (one usage of which is as a "phone") is hardly a limitation. But I guess it would depend on what you are trying to do. I have no idea what you would want to do. Only you know that.
As for what others might do, were they to be able to upload their own software to their phones, well, the only way to answer that question is to let them and see what comes out of it.
Only a fool would believe he could direct, let alone predict, all the uses that might be made of a particular software program, a particular language or a particular computer.
One use of a computer is as a communications device, aka a "phone". Another is a "game console". There are plenty of other uses for handheld computers even if you yourself cannot think of them.