There are a few reasons why there are shallow parts in the ocean. One reason is that the Earth's crust is not completely rigid. It can bend and flex under the weight of water. This means that the ocean floor can be pushed up or down by the movement of water. Another reason for shallow parts in the ocean is that the Earth's crust is constantly being created and destroyed. When new crust is created, it is usually pushed up from the ocean floor. This can create new landmasses or shallow water areas. When crust is destroyed, it is usually subducted, or pulled down, beneath the ocean floor. This can create deep ocean trenches.
The shallow parts of the ocean are important for many reasons. They provide habitat for a variety of marine life, including fish, shellfish, and seabirds. They are also important for human activities, such as fishing, shipping, and tourism.