As Yuval pointed out in the comments Haskell is based on the $\lambda$-calculus. Another reason why lambda calculus is so important is the Church-Rosser Theorem, which justifies the existence of lazy evaluation.

As Yuval pointed out in the comments Haskell is based on the $\lambda$-calculus. Another reason why lambda calculus is so important is the Church-Rosser Theorem, which justifies the existence of lazy evaluation. (Which is one of Haskell's many cool features)