Reason of button-down shirts have that little loop on the back???

According to fashion history, the loops were originally used by East Coast sailors. One excerpt reads that "sailors would hang their shirts on ship hooks when changing. ... The trend started with the US made Oxford button down shirts in the 1960′s."

Tthe Gant company started adding little fabric loops to the backs of their shirts They called them “locker loops”, because men could simply hang their shirt up by the loop in their lockers and keep their button-downs wrinkle free.