There is really nothing that makes it "necessary" - it simply a convenience for users and tool writers that regardless of distribution, they can find the device tree files from same place.

Back to track, it Seems everyone agrees in putting dtbs under /boot. The kernel make install location /boot/dtbs/$(uname -r)/ has been preferred (Alex, Paolo), with the other candidate being the fedora location /boot/dtb-$uname -r)/.

Riku