Because it's a quick way to begin understanding how a developer thinks (on their feet, under pressure, in an unusual coding environment, with someone watching t

Because it's a quick way to begin understanding how a developer thinks (on their feet, under pressure, in an unusual coding environment, with someone watching them). It's easier for the interviewer to observe than giving them a sheet of paper or watching over their shoulder. In those days online video-conferencing and whiteboard apps didn't really exist in any commoditized fashion.