As I've been browsing the forums I have noticed that many of you own more than one ereader. I understand reading on a phone and I understand the iPad, but for those of you who own Kindles, a Sony and various assorted other devices... why? Do you use them at the same time or is it a progression of newer and better devices?

Just curious.
(Or maybe I need to build the argument for my husband of why I would possibly need a second ereader?

)