It has got to do with fuel consumption - here is why. The velocity of the orbiter is not constant around earth. It is the lowest when the orbiter is closest to earth. Minimum velocity means the speed is reducing, comes to an inflection point and then the speed starts increasing. Acceleration which is change of velocity/time is therefore the highest at the inflection point. At this point, the thrusters are engaged at full blast. This increases acceleration of the orbiter even more than what it was going to be and this leads to an increased average velocity of the orbiter. Now increased average velocity is directly proportional to the radius of the orbit. So just by engaging the thrusters at the highest acceleration point in each revolution, the radius of revolution keeps increasing and eventually the orbiter reaches a specific distance between the earth and moon where the gravitational influence of both of the bodies are equal and opposite, meaning they cancel each other out. At this point, the thrusters are engaged again to slightly tip the orbiter towards the moon’s gravity well and the orbiter automatically starts a new trajectory around the moon. Again to reduce the radius of revolution around the moon, at the highest acceleration point (or when the orbiter is closest to the moon) the thrusters are engaged, only this time to reduce the average velocity around the moon, so in consecutive revolutions, the radius keeps decreasing.

The beauty of this is that, by making more revolutions around both bodies and engaging thrusters only at specific times, the fuel consumtion is reduced drastically, the flipside being that it takes more time. Remember that once a body is set in perpetual motion around another celestial body in space at a precalculated average circular velocity, it takes no additional energy to keep it in perpetual motion as the centripetal force (effect of gravity) remains constant and is not being counteracted by any other force. So when the average velocity of the system is increased, the radius of the circular motion has to increase