Just to hide the wires to look cleaner. Aw, I don't think I could have a case that didn't have room behind the tray after having one with it, it's so nice!
There are plenty of flipped ATX cases with space behind the tray. What I would do is mount your drive vertically in the front bay and zip tie the cables to the far side out of site.
I know they are out there though. Those things have been around years and never really took off meaning you can probably find some for real cheap if you dig around. Like an old TJ06 or something.