It is an opportunity for the horse to "Follow" the line or rope.
This idea transfers to the ride as the rider asks the horse to change direction (in the beginning) with a direct rein.
This is also a foundation for long line driving which also helps to solve problems when not mounted.
The work is accomplished at the walk or trot in most cases.
This is a 20 month old filly that is being worked and readied for driving.
Notice how the lesson starts and ends.