Think of an orange; it is not perfectly round but slightly flattened at the top and bottom. Similarly, the Earth is slightly flattened at the poles, making our 'average' radius a convenient simplification for calculations.

The Earth is not a perfect sphere due to its rotation, which creates a centrifugal force that causes a bulge at the equator. The value of $6400$ km is the standard effective mean radius adopted for most competitive exam physics problems, particularly when calculating acceleration due to gravity ($g = \frac{GM}{R^2}$) or orbital mechanics.

Remember 6400 km as a constant for almost all gravitational potential energy and weight problems in physics exams.