The value of 'g' varies primarily due to two factors: the Earth's shape and its rotation. The Earth is an oblate spheroid, meaning it is slightly flattened at the poles and bulges at the equator. This means points at the poles are closer to the Earth's center than points at the equator, leading to a stronger gravitational pull (higher 'g') at the poles. Additionally, the centrifugal force due to Earth's rotation is maximum at the equator and zero at the poles, which slightly counteracts gravity, further reducing the effective 'g' at the equator. Consequently, 'g' is maximum at the poles and minimum at the equator, making the weight of a body maximum at the poles.