Question

why ocean or sea of limited use to human beings?

Answer 1

Oceans are of extreme usefulness to humans: they provide the rain. Rain creates lakes and rivers.

Life would be difficult without this cleverly desalinated water.

Oceans also contain fish, which is useful. Oceans provide borders for continents, which assists with the variety of lifeforms. Change enables greater survival potential.

Seawater can be desalinated at large scale by countries with access to large quantities of cheap power but poor access to fresh water, to create fresh water in volume.