Hey there! I'm happy to help!
First of all, the seas and oceans provide fish, crabs, lobsters, and other sea animals which are very important to the nourishment/economy of the area. Many people living in Pacific islands, for example, might depend on fishing for food. Louisiana is famous for their crawfish, Maine their lobster, etc. Those are very important parts of their local economy.
Additionally, they contribute to a sense of place. For these areas. It is hard to picture Southern California, Hawaii, Florida, Bali, and other oceanside locations without the beautiful beaches and rolling waves. This is very important to people who live in and/or visit these areas.
Have a wonderful day and keep on learning! :D