Is there really any reason to have multiple devices besides collecting them for the sake of collecting them? Or for those who do own multiple functioning units, why do some sit while others get used?
Yes, and at least in my case its surprisingly simple... Laziness.
I don't want to have to put in effort every time I change flavors -- I have about 2 dozen flavors I rotate, and i'll puff on 5-7 throughout the day. I don't want to have to swap out parts every time I want a different taste, so I keep 2 Provari's with various rebuildables or dripping attys, and 2 VMod's dedicated to a couple flavors with me at all times. Then I've always got the variety I desire without interrupting what I'm currently doing for the sake of a new flavor choice.
As far as differences between units -- absolutely, from build quality to amperage limitations to power loss/overhead to model-specific options (if thats your thing) theres quite a range of difference. They all ultimately end with the same results, but there certainly are differences between models that can make a difference. Those lines become more blurred the higher up the scale you get, and at some point it becomes more about hobby status than functionality -- but that doesn't negate that the differences from a lower range, mid range, and high range exist.