The wide plain was a symbol of the large peace and silence which comes with the spiritual consciousness when one rests in the Divine.
It is in the silence of the mind that the strongest and freest action can come, e.g., the writing of a book, poetry, inspired speech, etc. When the mind is active it interferes with the inspiration, puts in its own small ideas which get mixed up with the inspiration or starts something from a lower level or simply stops the inspiration altogether by bubbling up with all sorts of mere mental suggestions. So also intuitions or action, etc. can come more easily when the ordinary inferior movement of the mind is not there. It is also in the silence of the mind that it is easiest for knowledge to come from within or above, from the psychic or from the higher consciousness.
By enforcing the peace of the higher being in the lower parts down to the physical it becomes possible to (1) create that separateness which would prevent the inner being from being affected by the superficial disturbance and resistance, and (2) make it easier for the Force and other powers of the higher being to descend. When one has gone so far that peace from above can descend, that is a considerable progress.