Scientists use the light year because nothing can travel fast than light. Therefore, in any given time frame, light will travel farther than any other substance or particle in the universe. For more precise measurements, scientists use astronomical units. One AU is equal to the distance between the earth and the sun. For anything less, scientists usually use kilometers. For extremely large, interstellar distances, light years are very useful. You wouldn’t want to measure the distance between Earth and Proxima Centauri in kilometers or even AU. The lightyear gives a simple, relative measurement.
