After learning the long form on the right side, John Kells then taught the mirror image form.
This is called the left side because we started by sinking in the left leg. Let me dispel straight away all the idiotic suggestions that this will make your Chi run backwards or somehow damage your health.
What was interesting for me was the effect it had in terms of coordination and the mental clarity it evoked. Even now it feels better and more natural to me to do a left side long form.
Whilst we will always favour one side more than the other, especially in applying the techniques, I do believe it makes sense to work both sides to increase mental agility.
The left side form also included some new postures and variations which we could then incorporate into our right side.
I don't believe it is absolutely essential to do a left side as we should thoroughly master one form and the principles and techniques. But it does, in my personal opinion, add interest and depth to my practice.
Of course, in the words of the late Patrick Swayze in Roadhouse...opinions vary.