Baitrunners were designed primarilly for live bait fishing.
The idea was to allow the bait to freely swim around & the target fish to be able to take the baitfish with little effort whilst still eliminating overruns/tangles that would occur when freespooling. That light drag prevented over-runs whilst also allowing the fish to take the bait & run until he was ready to scoff it down, at which time the main drag was engaged to fight the fish