Being closer to your situation than to that of U.S. citizens (I refuse to call them "Americans", I am an also an "American" but not a U.S. citizen), I can only guess, but I think some answers could be:
- Access to public domain books
- Access to free books offered by self-published authors, many of them only publishing digitally
- Access to free books written in languages other than english which are hard to find in brick and mortar shops
- Access to library ebooks
- Access to books not easy to find in paper but which have been recently released electronically
- Portability
- Having run out of space at home for storing pbooks
- Reading news