This passage, which unfortunately lacks references, about the development of ADTs in Haskell, from A History of Haskell: Being Lazy With Class, section 5.1:
In general, an algebraic type specifies a sum of one or more alternatives, where each alternative is a product of zero or more fields. It might have been useful to permit a sum of zero alternatives, which would be a completely empty type, but at the time the value of such a type was not appreciated.
leaves me wondering, how would such an ADT be useful?