Question

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

Answer 1

Actually it is really doesn't matter ,by which unit you're measuring the distance. It's depends totally on your convenience.

So come to the point , why it is convenience to measure in light year or parsec rather then in km.

Explanation :-

The distance from one Astronomical bodies to the other is very large. 
So it is the main reason of using the Astronomical units .
Like one parsec is About 30 trillions km. and one light year is about 10 trillions km.

So it is harder to say or calculate the values in 30000000000000 km (approximately) rather then in 1 parsec.
Centimeters , Meters , Kilometers are bit inelegant on compare to astronomical distance. It is also easy to remember and visualize.

Taking an example of Interesting Fact :-

The nearest star from the earth is Triple star system -Alpha century which is  almost 4.37 light year from the earth. 

1 light year = 9.5 trillions Kilometers .
Imagine saying or calculating the distance of Alpha century in Kilometer become harder to calculate on compare to light year or parsec.



Hope it Helps :-)