SSDs are traditionally marketed with sequential IO performance expressed in MB/s or GB/s, and random IO performance expressed in 4kB IOs per second (IOPS). Using larger block sizes for random IO will increase throughput in terms of GB/s but will almost always yield a lower IOPS number. Block sizes smaller than 4kB usually don't give any improvement to IOPS because the SSD's Flash Translation Layer is usually built around managing data with 4kB granularity.

> expressed in 4kB IOs per second (IOPS). Using larger block sizes for random IO will increase throughput in terms of GB/s but will almost always yield a lower IOPS number

and higher IOPs numbers give the marketing department the warm and fuzzies. Got it.

It is a good metric because it is a measure of random access. Bigger requests are a mixed measure of sequential performance and random access (and you can basically infer the performance for any io size from the huge request bandwidth and the smallest reasonable IO IOPS)