Because it's not easy for a horse to walk by itself in a proper LFS on a large circle in the beginning!
(LFS stands for Lateral bending in the body + Forward down tendency of head and neck + Stepping under of the inside hind leg under the center of mass. Click HERE for a video on What Does LFS Mean?>> ).
Therefore you have to start with LFS in hand and gradually you can increase the distance between you and the horse. This gradual proces gives you the means to support the balance, suppleness and shape of the horse with your aids from a closer distance when needed.
Here’s how to set up this gradual process:
To begin with, it's enough to ask the horse to walk about 1 meter (3 feet) away from you.
With some practice, this will later become 2 meters (6 feet) and then 3 meters (10 feet).
There will come a time when 4 meters (13 feet) is going well, and you can increase the distance again.
Do this until you have about 7 meters (23 feet) distance between you and the horse.
During this process, the horse will show you when he is ready to go further away from you and is ready to continue with LFS in self carriage, with less help from you.
So for example you go from 1 meter (3 feet), to 2 meters (6 feet), to 3 meter (10 feet) and at 4 meter (13 feet), your horse starts to lose balance, suppleness and shape.
Now the moment the horse starts to lose the LFS, return to a position more close to the horse.
So if the horse loses the LFS at 4 meters (13 feet) away from you, go back to 3 meters (10 feet) and slowly go towards 4 meters (13 feet) again.
Remember: only the moment the horse shows self carriage again, you can start to increase the distance again.
So to go from groundwork to longeing will be a gradual process, in which you will increase/decrease/increase/decrease the distance between you and the horse many times in the beginning.
During this process, strive for quality, not for a quick fix.
So take the time it takes to get a well balanced horse on a big circle!