You're right to ask that! While kilometers and miles are perfectly valid units for measuring distances on Earth, they become utterly impractical when dealing with the vast distances of galaxies.

Here's why light-years are used:

* Immense scale: Galaxies are incredibly large. The Milky Way, our own galaxy, is estimated to be about 100,000 light-years across. Expressing such distances in kilometers or miles would result in incredibly large and cumbersome numbers, making them difficult to comprehend and work with.

* Ease of understanding: A light-year represents the distance light travels in one year. Since light travels at a constant speed (approximately 300,000 kilometers per second), a light-year provides a convenient unit for understanding how far away objects are in terms of how long it takes light to reach us.

* Relativity: In the realm of astronomy, we often deal with distances so vast that the time it takes light to travel becomes a significant factor in our observations. Light-years naturally incorporate this time element, making calculations and interpretations easier.

Example:

Imagine trying to say "Our galaxy is 100,000,000,000,000,000,000 kilometers wide" versus "Our galaxy is 100,000 light-years wide." The latter is much more concise and digestible.

In essence, light-years provide a more manageable and relatable unit for measuring the immense distances found in the cosmos.