It is popularly used as a leavening agent for flat pastries. Useful note! Crush to use as smelling salts to revive a fainted person.
Baker's Ammonia predates baking powder and baking soda. Used first in Scandinavia regions to make light pastries and types of cookies. It continues in use today where required in old-fashioned recipes where crisp and light cookies etc. are wanted. Online, one reads that "Icelandic air biscuits" cannot be made airy and light with anything other than ammonium carbonate. The strong ammonia smell produced during baking will dissipate when cookies are done.