Stars are incredibly far away, so using traditional units of measurement like kilometers or miles would result in astronomically large numbers that are difficult to comprehend and work with.

Here's why we use light-years:

* Convenience: A light-year is the distance light travels in one year, which is a very large distance. Using this unit simplifies the representation of these immense distances. Imagine trying to say "the star is 9,460,730,472,580,800 kilometers away" versus "the star is 10 light-years away".

* Relativity: The speed of light is the fastest speed possible in the universe. So, measuring distances in terms of how far light travels in a certain amount of time is directly connected to the fundamental limit of how quickly information can be transmitted across the cosmos.

* Historical Significance: The concept of the light-year emerged as scientists started understanding the vastness of space and the speed of light. It became a natural unit for measuring cosmic distances.

Why not use other units?

* Kilometers/Miles: These units are too small for interstellar distances, resulting in incredibly large numbers.

* Astronomical Units (AU): These units are useful for measuring distances within our solar system, but become unwieldy for the vast distances between stars.

In summary: Light-years provide a convenient and relatable way to express the immense distances between stars, taking into account the speed of light, the ultimate cosmic speed limit.