A light-year is the distance that light travels in a vacuum in one Julian year (365.25 days). It's a unit of astronomical distance.

There are 1,000 millimeters in one meter.

Because a light-year is an enormous distance (billions of kilometers), while 500 million millimeters is a relatively small, terrestrial distance (500 kilometers).

Primarily for educational purposes and to emphasize the vastness of space by comparing an extremely small unit to an extremely large one.