If the formula_10 component were connected to ground, the filter would be a voltage divider composed of the formula_11 and formula_10 components cascaded with another voltage divider composed of the formula_13 and formula_14 components. The buffer bootstraps the "bottom" of the formula_10 component to the output of the filter, which will improve upon the simple two-divider case. This interpretation is the reason why Sallen–Key filters are often drawn with the op amp's non-inverting input below the inverting input, thus emphasizing the similarity between the output and ground.