John Duval, chief winemaker at Penfolds, adds it after fermentation to the legendary Grange Hermitage. But even in Australia, there's no consensus among winemakers. Some say it's only for unimportant wines, others will use it for big, structured wines. One point of agreement is that tannin is a major part of the structure that delineates mouthfeel in red wines. Wines convey a tactile impression, a sense of weight and texture.
Tannins are analogous to the beams and girders supporting a house.
Without that structure, the edifice of fruit collapses. But wineries need to exercise restraint. Just as brawny muscles wouldn't flatter a ballerina, wines of polish and elegance are probably not ideal candidates for substantial additions. When overdone, tannins can dramatically alter the style of a wine. In essence, it is a debate between purity and quality.