I always thought it was a length pre-determined by at the start of the operation in case soldiers needed to relay information to another unit using a method of communication which can be intercepted.
So, A wants to know where B is. B radios back, we're 30 klicks south-east. But depending on what they've agreed a klick is, it's a completely different distance. A klick could be 50 yards or 150 yards or half a mile etc. A then knows where B is but the enemy doesn't have a clue whether B is 30m away or 30 miles away.
If that's not how it's used then it damn well should be.