How is it not totally different?
People use Airbnb to rent apartments and people use GitHub to develop and share source code for software.
Let me be more precise: "how is github's app [existence] different than e.g. airbnb's?".
The grandparent asked a question, my point is that you could replace "github" with any other company that keeps both a web app and a mobile app:
> > Could someone explain to me why would someone use GitHub on their phones rather than on a desktop computer?
I think I don't understand the question; how is github's app different than e.g. airbnb's?