It allows full annotation of PDF files, which sounds dull, but it means I'm now genuinely paper-free.
I travel all around the world and laptops are heavy. As I need to scribble on documents, I once carried paper as well. Now, however, I can make notes using the app, and email the PDFs to people. It makes quite a difference to the volume of bags I have to carry through airports.
The app stores the files with my notes on. My PA then organises them, so when I need them again, I can find the last conversation I had with that country or team, with my notes.
Using Post-it notes on a screen never quite worked for me. This app simply lets you scribble on the screen and draw arrows, which I do with my finger, although there is the option to use a stylus.