They are called Cage Nuts... kind of a pain to work with i feel, takes extra time to put them in. Im not sure 100% WHY they are used opposed to threaded holes... possibly to give the end-user an option of 10/32 or the other larger option... maybe cheaper to manufacturer?
I know my dell racks use them.You basically insert the cage nut and then your screw goes throgh your equipment into the cage nut and it holds it in place. Pretty solid actually although they sometimes fall out while installing equipment. I've had to take a drill to them when a screw got stripped going in so thats how secure they are... i prefer pre-drilled 10/32 screw terminals opposed to cage nuts, as i don't have to spend money on cage nuts then (not much like $12 for 50 or so i think i pay but still)
^^No worries about stripping the threads on your rack and losing a U of space permanently when you have cage nuts...
This is exactly why they use the cadge nuts. if you strip out a hole on a pre-threaded rack you're SOL or you're stuck rearraging equipment/servers which leads to unwanted downtime.