Why light years are used to measure distances in the universe

Since it becomes quite difficult to write and interpret this figure using normal units of miles and kilometres, astronomers use light year to interpret it.

Light travels at an extremely fast speed. So using "lightyears" means how fast and far light can go in an entire year (a whole lot). So it's a very very large scale of distance considering how fast light travels. Overall, it's mainly used because the distance is so great that measuring in miles or kilometers is not reliable or as sufficient as lightyears

Ithink it is space that makes up 70 percent

5×^2+20-15 or -(5x^2-20+15)