Buffering is an important part to combos in the KOF series. It allows you to save previous inputs for use in later inputs, making many combos far easier, and making certain combos that were previously impossible, possible.
An example would be Terry Bogard's simple 2C 3C 6 236D.
This works by saving 2C 3C 6 as 236, while also doing the inputs. Directly after said inputs, 236D will take the buffered 236 input as 236236D, allowing you to execute a combo directly into his DM without much effort. Without buffering, doing 2C 3C 236236D would be extremely difficult, nearly impossible.
Another example would be King's typical Maxmode activation (2002UM), which is fairly difficult without buffering a bit.
With buffering : 6C 3D 214 BC
Without buffering : 5C 3D BC 63214C