Generally, we provide horizontal bars at outer layer and vertical bars at inner layer. This is due to the cracking of the concrete. Walls are constructed on slab/ raft foundation or strip foundation. The foundation of the wall is act a restrain in horizontal direction when it is hardening. However it is free to expand/shrink in vertical direction.
Due to the restrain in the lateral direction at base, stresses are develop in horizontal direction. Crack will appear in the surface of the wall. We provide reinforcement to carry the forces in immature concrete. These are the horizontal reinforcements, we generally provided.