Kabr was a Titan, and he did some really amazing stuff. Plus if there wasn't a wall it be much harder to fend of the enemies. You can't just rely on full offense to win a war, gotta have a good defense too. Otherwise while your out there in the wilderness, the darkness can just walk around you and kill off everything you're fighting for. Give titans some love, we are all contributing to the survival of humanity. At least lore wise anyways.