Answer :

conungi
the reason scientists use light years instead of kilometres is becasue a light year is the distance traveled by light in one year, and if we actually did the math (ill just give it to you)[tex]9.4605289*10^{15} meters[/tex] to put that into perspective a kilometer is 1000 meters or [tex]1.0*10^{3} meters[/tex]. if they mesured everything in kilometers they would have to use even bigger numbers than they already do.
ttstreb
A light year is also a theoretical distance, meaning that we do not actually have a tool for measuring it, like we do meter sticks. Also, think about the vastness of space. The light we see from stars are about 1/64 of the Milky Way Galaxy, if not less. So yes, light years greatly reduces the amount of numbers used to measure distance. Using kilometers would be much too cumbersome.