Many forms of land-based life depend on fresh water, which comes from rain. Humans depend on rain to fill aquifers. Rain also plays a role in shaping the landscape and bringing nutrients to the ocean.
Creatures that live in the ocean can survive on salt water, but organisms on land generally depend on fresh water for survival. Rain is the means by which fresh water is created; when it evaporates, water leaves behind its salt. Without rain, there would be no fresh water available, and life would have evolved in a different manner.
Humans depend on fresh water for survival. While it is possible to remove salt from ocean water through a process called desalinization, it is expensive, and the cost of transporting water inland is prohibitively costly. Rain fills aquifers, and it keeps rivers and lakes filled. Forms of life that depend on rivers would not have evolved without rain.
Rain has a number of effects on the environment. Many canyons and other natural features were shaped by rainwater eroding surfaces. The Grand Canyon was created by a river, which was fueled by rainfall. Many species of life in the ocean depend on minerals that flow from the land to the sea. Without rainwater, they would not be able to survive.