in the anime, Armin possessed one of many forbidden book that depict the what is on the outside world (him telling erin about the sea and how the ocean's water is salty,but was written off by erin as shenanigans ). there could be tons of remaining forbidden books that have loads of info outside world. its just dark ages BS to keep people from wanting to leave, even with the threat of being titan snacks. another system of control.
i'm guessing there might be a war going on with the hybrid and normal titans (claymore mainland battle with those dragon class vs yomi). and they need erin because he is one of them and they need more soliders or he's is a hybrid of a special class via thanks to his dad.
what i found was surprising was that the female titan didn't try to open up the wall after the 50m titan was revealed to be inside. i'm thinking these hybrid titans have an agenda with wall praying religious sect, since they want to keep the walls intact.
maybe the inner wall is protecting something special, what else would require titan walls?