Stupid question, what is the benefit of putting the PSU at the bottom of the case, rather than the top?
I'd imagine having it at the top would be better since it wouldn't heat the CPU/RAM/GPU.
One advantage is cable management, if you look at the PSU the fan is on the top and the Native cables are at the rear of the case allowing for them to be routed around the back of the MB tray. if it were top mounted the fan from the PSU would blow directly onto a HSF rising the CPU temp (maybe, jury is out on this one, I believe it does), the Native cables would be at the front of the case insuring the 20+4 cable would be completely exposed, and a rats nest of bundled cables.
This installation method assists by causing minimal air flow restriction within the case and keeps a system looking clean inside. Also any heat generated by the PSU is dissipated with the fresh air coming in from the two front fans.