AC feeds + PSU redundancy
A racked machine does nothing until you plug it in. Step into the rack and press E on the AC inlet on the device's rear; the cord drops into the left cable channel and the box comes to life. Every server, switch, and router in a floor rack needs at least one.
Bigger machines have two power sockets. Plug in both and the box survives a power supply dying inside it: one supply fails, the other carries the load, and you swap the dead one on your own schedule instead of during an outage. The catch is that running two supplies wastes a little electricity, about 25 W per machine, all day, every day. Cheap insurance on a box customers depend on; pointless on one nobody would miss.
Detailed explanation
A device mounted in a floor rack is not operational until at least one IEC C13/C14 cord is seated. No feed means no forwarding, no allocations, no billing, and no draw; it is intentional player state, not a fault, so nothing pages. Wall-mount gear runs off the wall socket and is exempt.
With two cords seated, a PSU component failure degrades the box instead of downing it: the surviving supply carries the full load while you schedule the swap. The cost is real: a 1+1 pair runs each supply below its efficiency knee and the second supply burns near-constant standby and fan overhead, modelled as a flat +25 W per extra connected cord. That overhead counts against the rack breaker budget and lands on the power bill, so cord up redundantly where the blast radius justifies it, not fleet-wide by reflex.