Perhaps a silly question

[Avernite]

That one is simple. Because if there is a Wall, it is far, far better to be fighting on the far side of it than your own. And your guys have to go to that side to do that fighting.

To expand on that:

Many walls are breached slowly by undermining; a direct assault on a proper wall is difficult. The way to keep undermining away is to sweep the area outside the walls with sorties. As you pick the time and place to sortie, it's still much better than fighting your enemy way out in the open where they may have the initiative, so the Wall still has plenty of value.

[kbrizzle]

I am ok with the Gates not being literal but more conceptual constructs since this is the way Harry sees things (very black & white), although the existence of a Gatekeeper seems to mean there are actual Gates of some kind.

An interesting notion is that Rashid is called the Gatekeeper, implying that there is traffic through the Gates from time to time & someone is required to watch over them (what the actual term Gatekeeper means historically) - otherwise why wouldn’t his title be Gatestopper or something?