Simpson's Rule is a way of approximating the area under a curve, using a parabola. It finds the area of a parabola(s) that passes through a set of points, and 'automatically' computes the area(s) using the formula.
This is why when using Simpson's Rule for parabolic figures, we acquire the exact area. However, using it on say cubic curves can over-estimate or under-estimate the area, depending on the extent of the concavity of each turning point (assuming there exists more than one), and where you take your approximation.