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.
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) to put that into perspective a kilometer is 1000 meters or . if they mesured everything in kilometers they would have to use even bigger numbers than they already do.
Ever seen how a glass window fogs up if you breathe onto it? That is an example of condensation: water vapour from your breath condenses into water on the glass.
to put that into perspective a kilometer is 1000 meters or 
. if they mesured everything in kilometers they would have to use even bigger numbers than they already do.