Why is the sun useful to Earth?

In short, the sun heats the earth with, of course, sunlight. It also allows plants to use photosynthesis, a process that feeds the plant.

Without the sun, the earth wouldn't be dark. The earth wouldn't be there. If the sun was no longer around, the milisecond it left, the earth would freeze. If it blew up, so would the Milky Way, our galaxy.