Why are light years used to measure distances in space?
The light year is used to measure distances in space because the distances are so big that a large unit of distance is required.
Distances in space are vast. The units of measurement we use from day to day are far too small to measure distances in space without adding a large number of zeros.
For example the metre was originally defined to be one millionth of the length of quarter great circle from the equator to a pole. This means that the circumference of the Earth is about 40,000 kilometres.
Now consider the distance from the Earth to the Sun. It is about 150,000 kilometres. The number is already getting big.
We added a new measurement the Astronomical Unit (AU) which is based on the average distance between the Earth and the Sun. This unit is good for measuring the distances between planets.
Now consider the nearest star from us Proxima Centauri. It is 40 trillion kilometres away which is a ridiculously large number. Expressed in AU this is 268,770 AU. However some stars are a lot further away and even AU numbers become huge.
The light year, as the name suggests, is the distance light travels in a year. It is about 10 trillion kilometres!
Proxima Centauri is 4.25 light years away, which is a much more manageable number.
Hence the light year is a convenient and manageable unit of distance for measuring the distance between objects in space.