As a lodge-based system, the Golden Dawn is essentially crafted within the framework of Freemasonry, with an additional layer of magic placed upon those foundations. These Masonic roots are not hard to find, with everything from the officer structure to the grip, step and passwords of the grades stemming from earlier tradition.
Some of these elements have an obvious function, where their symbolic purpose and magical application was explained in detail. Others may appear to have no purpose at all, which has led to some modern magicians calling for them to be axed from the rituals. The Past Hierophant is the classic example of this.
On the surface, the Past Hierophant appears to do very little. He or she sits to the left of the Hierophant on the dais, watching the proceedings, contributing to the visualisations and inner workings of the ritual.
One important duty of the Past Hierophant is to build up the godform of Aroueris upon the Hierophant when he or she is moving (for the godform of Osiris never leaves the dais).
However, the Past Hierophant is also important for another major reason. The entire Hall of the Neophyte is mapped to the lower half of the Tree of Life, making every placement symbolically and magically important. The dais officers are not mere observers, but mark the places of five paths, with Past Hierophant on the path of Nun.
This can be seen clearly in the floor plans that were given to members of the original Order and its offshoots. I include a sample of one of these surviving documents below, while Crowley gives a crude example in The Equinox.
This alone makes it pivotal that the office of Past Hierophant remain, because without this officer we would be axing a path on the Tree of Life. Reshuffling the other officers is no solution, because they stand on important paths of their own. There are five upon the dais for a reason.
Part of the beauty of the Golden Dawn system is that nothing is arbitrary. Everything has a purpose, and even though we may not know that purpose initially, with study and practice we may uncover it in time. The consistency and coherency of the system is dependent on every element within it, so moving or removing one can easily upset or unsettle the others, and lead to other repercussions.
For those interested in delving deeper into the Masonic roots of the Golden Dawn, I highly recommend getting a copy of Frater YShY's forthcoming book Adept Magic in the Golden Dawn Tradition, which includes some interesting material in this area.