When construction of Bann'donor first began, its purpose was to prevent outside observation and keep enemies out. Once fully activated, it could also prevent enemies within its boundaries from casting spells, a function modeled after the Ley Line System.
It was a virtually unsolvable defense: to shut down Bann'donor, one had to extinguish the Sunwell; to breach the Sunwell, one had to force a way onto the Isle of Quel'Danas through Bann'donor.
The Ley Line System was much the same. As the most crucial installation Aman'Thul had left behind, its most important functions were protecting Azeroth against outside invasion and maintaining the world's stability. Its other offensive capabilities and energy transmission were merely incidental.
The more powerful a being was, the harder it was to force entry through the ley lines. Back then, Sargeras had forcibly relinquished most of his strength to enter them, so after entering Azeroth, he could only replenish himself by hunting dragons, only for Aegwynn to take advantage of his weakened state.
Probably only a being beyond a true god could break the Ley Line System by force from outside. After all, ever since Aman'Thul had established it on Azeroth, the world had become much safer, with Old Gods and great demon lords no longer descending at will.
Together with its source of energy-the Well of Eternity-the Ley Line System was likewise a virtually unsolvable defense: to force entry into Azeroth, one had to destroy the Well of Eternity; to destroy the Well of Eternity, one had to force entry into Azeroth.
Both were closed loops, their causes and effects contradicting one another, with no way to break them.
In Bann'donor's case, unless a being like Archimonde or Kil'jaeden forcibly broke it with their immense strength, anyone who entered Eversong Woods would merely be awaiting annihilation.