The Demon Beasts Formation was different, but the principles were the same. Hold the enemy back and warn the Demon Beasts if someone tries to attack and cross it.
One might be imagining why Rean didn't rush inside through the Demon Beasts' Gap after the armies broke the formation back there. Well, it was quite simple. The Divinity Realm experts didn't join the battle. That was something that probably wouldn't happen for a few days. Even the Divinity Realm Experts who had taken the front to attack the formation at the start immediately went to the back after the formation came down.
You wouldn't use your best fighters at the start. To be honest, Rean was one of the very few Space-Time Realm Experts who stayed at the forefront when that part of the protection formation collapsed. Naturally, that was supposed to happen since he needed to be struck down, which happened.
