Before buying an actual e-reader, I was already reading a lot on the computer. I've been reading fanfiction and copyright-free or out-of-print material on the internet since I was in high school (I'm 29 now).
When I got a smart phone, I started reading on that, too--mainly classics and freebies from FeedBooks or ProjectGutenberg. I won't lie; I wasn't actually keen on a proper e-reading device for a very long time. I thought, I have computers and an iPhone. Why do I need another gadget?
But when I started using my phone more and more for reading, especially at night, I would wake up the next morning feeling liked I'd scorched my eyes.
At my dad's urging--he bought the original Kindle when it first came out and loved it--I finally bit the bullet and bought the Kindle 3. No regrets. So much easier on the eyes when you intend to read a lot.
Also: If the books you read might cause you a sense of embarrassment (to purchase or be seen reading in public), any e-reading device is excellent. If you love trashy romance but hate those covers ...
tl;dr -- Clean answer: I bought a Kindle to read without eye strain. Less clean answer: I bought a Kindle to read smut in public surreptitiously.