Question

why is the 'light-year' used to measure distances between celestial objects?

Answer 1

it is a unit of measurement by the astronaut in space

Answer 2

The distances which we measure in the outer space are huge. The distance on the earth are relatively small so we measure all the distance in Km, miles or meters. To give you a simple idea the distance from earth to moon is roughly: 384,400 km (or 384,400,000 m) and relative to that, average human height is 1.6 m.
1 light year = 9.4605284 × 10^12 kilometers
This is a distance covered by light in one year. That is a H U G E number.