What can you do with root access as a developer? [on hold]

I have just started developing apps for the Android operating system (after years of iOS development) and I see all kinds of apps allowing you to do some pretty cool stuff using root access.

My question is, what is it exactly that the root access allows you to do, as a developer? Can I hook into other apps? Can I use functions of other apps installed on the phone (such as allowing to schedule Whatsapp messages)?