As my eyes continue to age faster than the rest of me, the format matters more and more. Reading ebooks on my computer allows me to use larger, clearer fonts, better colours, and to zoom in on diagrams and maps. Beats the hell out of using a magnifying glass to read paperbacks.
Reading ebooks on my computer also make it much easier to supplement my reading with the use of other resources. For example, when reading history I very frequently make use of Google Earth to take a look at battle sites, Wikipedia to take a side-trip to an article about something mentioned in the ebook, etc.