All my devices have cases because:
* I drop things. I don't mean to, but I do.
* I don't travel as much as I used to, but we've taken many road trips and airplane flights in the past and a case is so nice when you've got to put your Nook in one of those scanner trays at airport security, when you just need to toss it quickly in your carry on, etc.
* My wife and I both have the same make/model of e-readers so having different cases is really nice when you want to tell at a glance which one is on the coffee table and which one is the one you still can't find because maybe the cat knocked it off the table.
* I commute by bicycle, in all sorts of weather, and it's nice to know my e-reader (and phone) are covered in a case during uneven travel conditions.
* I've had my phone come out of my jersey pocket while riding down a hill and the phone bounced and skidded about a hundred feet before coming to a halt. Thanks to the case, it still worked, and though the case looked like it'd gone through the ringer, the phone was (and is) still fine.
Caveat: I like a case that my phone/ereader "wears" all the time, like a snap-on cover that you just flip back to use, I don't like a case where you have to extract the device to use it. For me, that just means I have two things to keep track of and/or lose, rather than just one.