The Boxes arrive, does it work?
Parts can be DOA, not work together. Returns have time limits, and building the final system into a case can be a waste of time if something doesn't work. Instead, build the rig up from the motherboard and power supply on a table, save all the packaging for RMA.
Cases provide airflow, cooling, and prevent damage to parts. Put an external fan on the board, or only use it for short periods of time. We show that at the end, but for those who don't read everything, like us....use a cheap external fan on the board.