It allows you to turn “overdrive” off when you’re trying to pass another vehicle on the freeway, so the van will stay in 4th gear as you stomp on the gas, instead of upshifting to 5th gear.
It’s also helpful if you’re driving down a long hill to turn the OD off, which again prevents upshifting into 5th and allows the engine to help brake your descent, taking some of the load off the brakes.