When I was comparing apps, DoubleLauncher was the only app that let you assign a true "page up/page down" to a hardware button.
Unlike the scroll up/scroll down that most other apps include I wanted a page up/page-down that functions just like the page up/page down on a desktop/laptop.
For me it makes one-handed web surfing and ebook reading a joy.
If the other apps still haven't caught up to include this invaluable function then they still have to take a backseat to DoubleLauncher.

Outside of a dog -- a book is a man's best friend.
Inside of a dog it's too dark to read.