Here's why:

* Vastness: The distances between stars and galaxies are simply too vast to be measured in familiar units like kilometers or miles.

* Light's Speed: A light-year is the distance light travels in one year. Light travels at a phenomenal speed (approximately 299,792,458 meters per second).

* Convenient Unit: Using light-years allows scientists to express these immense distances in a more manageable way.

For example, the nearest star to our sun, Proxima Centauri, is about 4.24 light-years away. This means it takes light from that star 4.24 years to reach Earth.