In today's world, we need the exact things wherever possible. That is the secondary reason. 

The primary reason is as follows: The length of a "metre" was defined ages ago.
The length of a "metre" was defined as per the length of an object (I don't remember what it is) kept at International Union Of Pure And Applied Physics (IUPAP). 

When the scientific revolution started with Galileo and Newton, the concept of absolute time was there. Thus people believed that speed of light was to be measured in relation to some other substance with respect to absolute time.

However we had to change the ideas of space and time when Einstein proposed the General Theory Of Relativity. The concept of absolute time is now gone. Now the concept of absolute speed of light is there. Thus any two observers would record the same speed of light, but different times acoording to their position.

Now it is found that the distance light travels in 1/299792458 s exactly corresponds to the length of the object I mentioned before.

Hope this helps you and all the other people who read this. 
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