Answer:
The universe is vast and so much of it still remains unexplored to this day. The units and references we use for distances and sizes down here in our lives (on earth) are not adequate to describe and understand the objects in outer space. Hence, the system developed to measure or map distances of astronomical bodies can be extended to other parts of the universe as well - since it will be consistent with that as well.
Hope that answers the question, have a great day!
