Question

The speed of light is used to measure distances in space because

light travels best in space

light travels really fast

the distances are so immense

the distances are unknown

Answer 1

The distances are unknown

Answer 2

agree with GB above. Light travels at the speed of light (tautological, I know) which is 300, 000, 000 (three hundred million) metres per second. According to Albert and Mileva Einstein, nothing travels faster than that speed.
So, using light, and a clock or other timing device, and the CONCEPT of speed as distance divided by time, then having got the speed sorted out and having the clock, a TIME OF FLIGHT technique can be used to measure a distance.
When the distances become very large, the idea of a LIGHT YEAR can be used. It seems that the Milky Way galaxy of stars (the one "we" are in) is 100, 000 light years across. A light year is the distance that light travels in 1 year, where the speed of light is in metres per second, and the time is the number of seconds in 1 year.

I think that this is roughly how RADAR (RAdio Direction And Range) works.

(ref "Special Relativity", AP French, MIT)