In 2016, researchers from the University of Illinois left 297 unlabelled USB flash drives around the university campus to see what would happen. 98% of the dropped drives were picked up by staff and students, and at least half were plugged into a computer in order to view the content. For a hacker trying to infect a computer network, those are pretty irresistible odds.
USB devices have been around for almost 20 years, offering an easy and convenient way to store and transfer digital files between computers that are not directly connected to each other or to the internet. This capability has been exploited by cyberthreat actors, most famously by the Stuxnet worm in 2010, which used USB devices to inject malware into the network of an Iranian nuclear facility.