Both malic and citric acids are used to acidulate other fruits (commercial wines can legally only add acids that occur naturally in any particular fruit).
This acid is found almost universally in temperate fruits. It dominates in apples and together with tartaric acid accounts for most of the acid in grapes. The main disadvantage of malic acid is that it buffers to a fairly high pH.
The form of commercially available malic acid added to wines is not subject to M-L fermentations.