The 10mm auto is a curious cartridge.
Designed originally as a best-of-all-options for the defensive pistol world, it was targeted to be an all things to all people service pistol cartridge. Sort of a hybrid of the service pistol standards, .45 ACP and 9x19 rounds. The goal? To have more capacity than the .45 and be more powerful than the 9mm.
Without completely retelling the detailed history, in the early 1970s, the late Jeff Cooper was reportedly looking for a round that combined the advantages of both velocity and momentum. The ballistics of a 200 grain .400 (10mm) diameter bullet traveling 1000 feet per second looked good to Jeff on paper.