The magnitude of a vector v is calculated as Mathf.Sqrt(Vector3.Dot(v, v)). However, the Sqrt calculation is quite complicated and takes longer to execute than the normal arithmetic operations. Calculating the squared magnitude instead of using the magnitude property is much faster - the calculation is basically the same only without the slow Sqrt call. If you are using magnitudes simply to compare distances, then you can just as well compare squared magnitudes against the squares of distances since the comparison will give the same result.