This practice was started by some sommeliers who tried to increase the sweetness of a “demi-sec” champagne by reducing bubbling.
Paris is still the city where this is most commonly done so, and this approach has spread throughout the world, and for almost all varieties, styles and ages of champagne.
Benefits of decanting champagnes are the reduction of the larger bubbles and a more agressive bubbling. That means that the best champagnes to be decanted are the younger, cheaper and non-vintage cuvées.
But it is also important to mention that many more expensive and well-known champagnes can benefit from decanting, too.
A pointer from Tom Stevenson, one of the world references in this field, says that the predominantly oxidized champagnes become more fresh after they are decanted.