Adobe ebooks checked out through libraries have the return date baked into the book, and the book becomes unreadable once the loan expires.
Kindle ebooks checked out through libraries will be automatically deleted when you connect to WiFi and Amazon's servers indicate the book has expired.
You might want to buy your books if you don't want to have to wait for your reserve (just like paper books), or if you want your own, permanent copy, or if the library doesn't have the book you want...
... basically, just like how libraries have always worked.
