Malolactic fermentation transforms malic acid into lactic acid.
This fermentation takes place at the end of alcoholic fermentation.
Like all fermentations, it develops the wine's aromas.
It is not compulsory.
Some Champagne operators prefer to avoid it to preserve the freshness and floral, fruity aromas of the grapes.
But most prefer to trigger it to obtain softer, riper, slightly milky aromas.