Seawalls help to maintain everything on both sides of the wall, including the land and the water. It is important to keep the soil dry by the wall or pressure can build up and cause the wall to buckle. Wet soil can also lead to sinkholes. Any structures near the seawall benefit as well.
One purpose of the seawall that is also overlooked is that it helps to protect the water. Some seawalls maintain the depth of the water, allowing for activities, such as boating and swimming.