You need a visualization of the outside obstacle and what can be better than a wall. For the Palestinians it means a division from each other, because the wall didn't separate Palestinians from Israelis, it separated them from themselves. This is the reality, and the wall is a kind of jail to the Palestinians.
I like movies that deal with trapped men. Men that need to make choices that are not obvious or easy choices. Then how do you visualize this? You create this character conflicted between two sides, because drama is about the conflict of two things, between your duty and your will, between what you want and what you can't have. It is all conflict between two things, and this is why you put your character in a place where you can visualize the conflict.
Filmmaking in general is about feeling and not about theory. You need to know a lot of rules about filmmaking: character development, grammar, and all these thing, but then you use it instinctively. I ask myself this question all the time. I have no solid theory, I just do what I feel is right.