I think I had several reasons. I like to read, and getting the occasional free book that I can read without being tied to a desktop or even a netbook computer is a plus, as is the lack of eye strain that LCD screens give you after a while. Reading whatever I want without anyone knowing what I'm reading might be a plus too.
And I like gadgets that I can use anywhere. I hadn't thought of pbooks being 14th century tech til I read the OP's posting. lol. Carrying as many books as I want in the palm of my hand is a definite plus though too.