What is rain useful for? why does it rain?

Rain is useful for watering plants and grass and provide water to animals and fill lakes in and stuff

Though Rain is a part of the water cycle and it's important for all the reasons that the other answerers have mentioned there is also something else about rainfall that the others have left out. Rainfall is the condensation of water vapor that has generally been heated by the sun. As such there is potential energy in it. All of the hydroelectric power in the U.S. is an indirect result of rainfall. The leveling of mountains is the indirect result of rainfall. Civilizations like Egypt flourished because of the fertility of the Nile valley caused by seasonal flooding which was in turn caused by rainfall. The Grand Canyon is an indirect result of rainfall.

When water becomes warm enough, it evaporates as vapor into the air. When a mass of air quickly cools to its saturation point, the water vapor condenses into clusters of tiny water droplets and frozen water crystals. We call these clusters clouds. Over time, the droplets and crystals that make up a cloud can attract more water to themselves. When water droplets grow heavy enough, gravity pulls them down as raindrops. If the air is cold enough, the ice crystals can remain frozen and grow large enough to fall as snow, sleet, freezing rain or hail.

Its in the national history museum and you can't se the original because its locked away in a vault 

Well, a and b together would be negative. b alone would also be negative.