It's not well documented online, but if you use Dash on Mac, then you can set up an iOS device as a 'Remote', which lets you use your phone/tablet as a second screen for documentation, which you control from your Mac. I think that's the primary usage scenario.
I occasionally use it on my iPad as an extra screen. It's also useful while on a plane. The desktop version if coding, or iPad if I just want to browse docs to learn a new API or something.
In which situation do people read documentation on their phone ?