-Let's say I'm looking up how control flow works in a programming language I have little experience with. I can skip to the index, find control flow, and click on that in a few seconds.

-The current chapter covers a new technique that I think could be applied to a previous example, and I want to check that example out. With the iPad/PDF I can easily move back and forth, with a Kindle I can't.

-The current page references a technique from a few pages ago where I don't remember the exact details. I can easily scroll back and forth with an iPad, not so much with a Kindle.

-I want to see if a book covers a relatively obscure topic. The iPad or my computer let me do a search through a PDF for keywords and preview each section containing the keyword, a Kindle doesn't.