Think about waiting somewhere in the middle of the day, or perhaps in an airport while you're traveling, and having immediate access to 100,000 books and hundreds of newspapers and periodicals from this pad in your hand.
Being at home on a rainy day, but wanting to read some new novel that just came out, you can have it in seconds. This concept (not hobbled by DRM) is pretty damned liberating.
As glued to the net as we all are, there's vast quantities of content that is not available online.
Typically ebook readers are larger than PMP's with larger screens. If you're going to read for a few hours, then the screen of a typical PMP becomes pretty eye-straining (due to size, contrast, and brightness).