Buckshot without buffer media spins as it travels along the bore and out the muzzle. This is both because of barrel vibrations and the large size of the shot. Birdshot also spins, but birdshot loads, because of the smaller pellet size, are more fluid and are less effected by the spin than buckshot. Unbuffered buckshot leaves the muzzle spinning with precessive force, which curves the flight, making for inaccurate loads or loads with too wide a spread.
Buffered loads have reduced spin, and also, reduced deformation, so they have better ballistics and make better patterns. Buffered loads in combination with plated shot usually make the most accurate and best patterning loads.
If you handload your shotshells, and wish to use a buffer, be sure to use loading data for the kind of buffer you're using. Buffers change the fluidity of the load, and can greatly affect pressures.