For a long time after the Kindle was released, I continued to read paper books only. I enjoy making notes in the margins, so I can easily review a book after I've read it by skimming my own markings.
Fast forward a couple years, and Amazon allows highlights in Kindle books and will sync that information online. I tried it, and ever since, I opt for Kindle books over paper books for two reasons: 1) ebooks take up less space than paper books 2) I can use the Ruby .gem kindle-hightlights to access my account and programatically retrieve the books I've read and the my annotations within those books.
Having my book annotations available online is a valuable resource for me, and I like that I'm able to share content that I've enjoyed with others.