Rami Arieli: "The Laser Adventure" Chapter 9.3.1 page 1
9.3.1 Laser Range-finder

Measuring distances with high speed and high accuracy was the immediate military application after the laser was invented.

Since the laser beam is electromagnetic light, it is traveling in space with known velocity (the velocity of light c).
By sending a short laser pulse to the target and measuring the time it take the beam to arrive at the target and reflect back to the sender, it is easy to calculate the distance.
Measuring distances with high accuracy is important for military applications such as:

Numerical Example:

How much time will the laser pulse travel, when it hit a target at a distance of 1.5 kilometer?

t = s/c = 3,000 [m]/3*108 [m/s] = 10-5 [sec] = 10 [micro-sec]

This time is well within the response time of standard electronic equipment.

To play with an interactive simulation of a range finder, click here.