Here's why:

* Vast Distances: The distances between planets and stars are incredibly vast. Using standard units like kilometers or miles would result in extremely large numbers, making calculations cumbersome and difficult to comprehend.

* Relative Scale: An astronomical unit is defined as the average distance between the Earth and the Sun. This means that distances within the solar system can be expressed as multiples of the Earth-Sun distance, making it easier to grasp the relative sizes and distances.

* Historical Significance: The AU has a long history in astronomy, dating back to the time when astronomers could only measure distances within the solar system relative to the Earth-Sun distance.

* Practicality: For many calculations in planetary science, using AU is more convenient. For example, Kepler's Third Law, which relates a planet's orbital period to its distance from the Sun, is most easily expressed in terms of astronomical units.

Example:

* Saying that Mars is 1.52 AU from the Sun is much more concise and understandable than saying it's 228 million kilometers away.

However, AU is not used for distances outside the solar system. For interstellar and intergalactic distances, scientists use light-years or parsecs, which are better suited for measuring the vast distances between stars and galaxies.