Building giant walls around your city rpg has obvious benefits to most hosts. Now from what you have said so far it doesn't seem that you really care about other points of view, so even though I'll try to explain it to you the best advice is to
ignore anything in this thread and lock it, then just host the city rpg how you want to. Now that that is out of the way here are three common reasons to build the walls.
- Forcing people to play, anyone greifing, spamming, or afk can quickly be found without admin powers.
- Reducing the possibility of people creating spam somewhere you can not see.
- Keeping people in a preset area.
Now you should have noticed that these three things have little to do with the actual city build. The walls are mainly there as an extra way for the admin(s) to exert control over people who join their server,
because we all know that the world will end if someone doesn't listen to you.