Buffer

A buffer converts a high impedance signal into a low impedance signal without changing its tone (hopefully!). This allows you the ability to use long cables or large amounts of true bypass pedals without capacitive tone loss, and also plug into effects that have a low or non-ideal input impedance (think vintage fuzz face). Sometimes they're sonically worthwhile - sometimes they're sonically detrimental. In the end it's aurally up to you as to whether they're needed in your setup or not!