I think it’s for the fit and looks. Probably likes a lower front end but flat bar doesn’t look enduro enough.
I like the look of a riser too, just not the look a negrise stem. I've wondered if a flat bar and riser with the same sweep etc. would feel different at the same hight because of where the bars interact with the steer tube. Probably negligable though.
All other things being equal it should feel the same.
I'd perform the experiment myself, but i'm too broke. Could someone with a lot of disposable income give it a whirl and holla back at us? I'd love to hear about it...