You only need one of those phillips-to-hex adapters ("circular things") to install your brass standoffs.
You screw your standoffs by hand into the motherboard tray where your motherboard holes are, use the "thing" to tighten each standoff, pull the "thing" off the standoff, put it on the next, tighten it, rinse and repeat. Once you have tightened all your standoffs, you put the "thing" away in your toolbox or wherever you put your tools and spare parts.
The 'thing' spares you the trouble of hunting down a socket wrench since the manufacturer knows you will already have a Phillips screwdriver nearby to screw the motherboard screws into the standoffs.