The damage isn't to the sloping foundation/walls, it's to the 'ramparts' on top of them. When a section of wall is destroyed, the 'ramparts' in that area are destroyed. Damage is visually represented by damage to the ramparts. This prevents defending units from 'docking' at that point, which makes it easier for units to scale the wall at that point. It also removes the extra missile protection that the ramparts provide to any docked unit. Because the foundation/walls themselves aren't destroyed, this action doesn't have the same level of significance that it did in games like RTW or M2TW; units still need to climb the walls to get in, so they're still more dispersed than they would be rushing through a breach in one of the other games.