I'm not here to attack or defend to-do apps, but regarding your statement, the utility of software rarely comes from the specific things it can do, but how it does them. There's usually non-functional differences.

What can be done with Python that can't be done with assembly? Nothing, but (for most cases) you can do it quicker and easier.

What can be done on an iPad that can't be done with a desktop computer? I doubt there's anything that you can do on an iPad that couldn't be done, in principle, on a desktop computer. The difference is portability and stuff like that.