Of course, it's not necessary to use a functional programming language to use these techniques. Because ideas from the functional programming world are appearing in mainstream languages, it is more important than ever to understand these techniques. Tom Christiansen said it best:
A programmer who hasn't been exposed to all four of the imperative, functional, objective, and logical programming styles has one or more conceptual blindspots. It's like knowing how to boil but not fry. Programming is not a skill one develops in five easy lessons.
Many programming languages offer a mixture of styles. Most object oriented languages have an imperative core, where classes, objects and methods provide a thin veneer over a language that is little more than a slightly improved version of C. Many functional programming languages mix functional, imperative, and object-oriented styles together in a manner that makes it difficult to tell them apart.
Haskell, on the other hand, is a purely functional language that restricts itself to the functional style of programming. Learning and using Haskell makes it easy to see the power and benefits of lambda calculus and functional programming.
"If you want to eat hippopatomus, you've got to pay the freight." -- attributed to an IBM guy, about why IBM software uses so much memory