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

why is it convenient to express the distancek of stars in terms of light year rather then in metre or kilometre ?

Express one parsec in terms of light year.

Express one parsec second in terms of light years.

Parsec is how many light year?

Express the average distance of the earth from the sun in parsec.

Express the average distance of the earth from the sun in light year.

Define Astronomical unit, light year and parsec.