Here's why:

* Newton's Law of Universal Gravitation: This law states that the force of gravity between two objects is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centers.

* Orbital Mechanics: When a moon orbits a planet, the centripetal force keeping it in orbit is provided by the gravitational force between the planet and the moon.

By knowing the following:

* Orbital Period (T): The time it takes for the moon to complete one orbit.

* Orbital Radius (r): The distance between the moon and the planet's center.

* Gravitational Constant (G): A fundamental constant of nature.

We can use Kepler's Third Law of Planetary Motion and Newton's Law of Universal Gravitation to derive the following equation:

M = (4π²r³)/(GT²)

where:

* M is the mass of the planet.

Advantages of using moons:

* Moons provide a natural, orbiting test mass. This allows us to apply Newton's Law of Gravitation more directly.

* Moons are often easier to observe and track than other objects. Their orbital periods and distances are more readily determined.

Note: This method is not always possible. Some planets don't have moons, and in some cases, the orbital parameters of existing moons may be difficult to determine accurately. Other methods, like observing the gravitational influence of a planet on nearby stars or analyzing the wobble in a star's motion due to a planet's pull, can also be used to estimate a planet's mass.