What might a concrete advantage of every piece (except Vaos) being able to move behind the pawn line in the setup be? [edit: closest thing to that I can think of, so far, is that a player sometimes just might be able to evade an attack/skewer by an enemy Vao or Cannon, in a way he couldn't otherwise]
Along the lines you imagined, it makes valuable pieces less prone to capture by pieces that can hop or leap over the Pawn lines. Since the Vao is the least valuable piece, it is the piece least in need of having an escape route. Notably, Chinese Chess, which introduced the Cannon, leaves more space around the pieces than Chess does.
Along the lines you imagined, it makes valuable pieces less prone to capture by pieces that can hop or leap over the Pawn lines. Since the Vao is the least valuable piece, it is the piece least in need of having an escape route. Notably, Chinese Chess, which introduced the Cannon, leaves more space around the pieces than Chess does.