Drag brake on a brushless motor was initially designed to simulate the drag created by the brushes pressing on the commutator in a brushed motor. It provides a subtle slowing when off-throttle. This can be useful when racing as the racer doesn't have to actively hit the brakes but can instead can benefit from a customizable drag brake effect to achieve a less dramatic slowing effect when setting up for a turn. (Just as one example.)
Additionally, on a crawler, drag braking can prevent the car from coasting back down the hill/obstacle when you release the throttle. Actually, brushed motors can also have adjusted drag brake settings-- Hobbywing's 1080 brushed motor ESC features adjustable drag brake settings. When driving my Temper over an obstacle I can release the throttle and the car stays put-- useful when trying to pick my way over and through a tricky obstacle/terrain feature.