Actually the question should be... why is the lambda calculus such a great programming language in practice? The lambda calculus is unique among other theoretic

Actually the question should be... why is the lambda calculus such a great programming language in practice? The lambda calculus is unique among other theoretical models of computation (automata, turing machines, etc) in that it is the syntactic and computational basis of several popular languages (lisp, Haskell, ML, etc). In all these languages, you can essentially write lambda calculus terms in a way that you cannot write turing machines in imperative languages.