As an example, you are approaching the airport from the west and landings are on runway 27. You’re on radar vectors and ATC has you descend to 3,000 ft when you’re 10 miles to the west to get you below the departing traffic (an inefficient plan but one that does occur from time to time). The base leg turn point is 8 miles from the runway, meaning you have over 20 track miles to go. That’s a long distance to fly with slats / flaps deployed to stay below 200 kts. The intent is to allow you to stay at minimum clean airspeed until you start extending slats / flaps for the approach.