Command queueing allows the device controller to handle requests out of order. Since it has intimate knowledge of the hardware it can order requests for maximum throughput.
If you benchmark an SSD with and without queueing you get very different speeds; with hard drives there is still a difference, although not as much.
EDIT benchmarks (Intel SSD)
no queuing, 4k reads: 3965 iops depth 4 , 4k reads: 16,900 iops depth 8, 4k reads: 25,700 iops depth 16, 4k reads: 32,600 iops depth 32, 4k reads: 38,800 iops
The benefit decreases with larger reads, but still exists.