This winemaking technique was originally used in regions where grapes ripened during a cool growing period and were hence picked with too much acid. While this is desirable in certain wine styles, such as sparkling wine, in table wine the results can be overly tart.
One solution is to add sugar to the wine through a process called chaptalisation named after Jean-Antoine Chaptal, Minister of the Interior in France around the turn of the 17th century. This is a somewhat inelegant remedy and indeed is illegal in some countries.
Instead, some winemakers will encourage the growth of certain Lactobacillus species that convert malic acid into lactic acid. The effect lies with the fact that lactic acid is roughly half as sour as malic acid, reducing the apparent tartness of the wine.
So called "new world" wine regions, such as California and Australia also use malolactic fermentation, although they rarely have trouble getting their grapes to full ripeness. The idea here is an attempt to increase the complexity of their wines, but sometimes results in buttery, cacky Chardonnays.