Why is it convenient to express the distance of stars in terms of light year (or) parsec rather than in km?

Explanation

The distances of astronomical objects like stars, planets etc from the earth are huge. The distance on the earth are relatively small so it can be measured in km.
For example:
The distance to be next nearest big galaxy Andromeda is 21,000,000,000,000,000,000 km. i.e. `21xx10^(18)km`
This number is so large that it becomes hard to write and to interpret.
So astronomical units like light year, parsec A.U are used for large distances.