Availability. In two directions.

One, not everything is available on Kindle, so it's worth asking for suggestions from the subset of materials that are available.

Two, local and immediate availability. Kindle greatly increases convenience by allowing you to have a selection of books with you much more often than you would otherwise. This can (and in my case has) lead to more reading than you might otherwise have done.

Kindle is one of those things that's made a bigger difference to my habits and choices than I expected prior to owning one.