It's easy to think that lambda calculus is not much more than a bizarre, impractical form of abstract math, but it is absolutely foundational to programming, not only providing a theoretical basis but also a rich set of simple, practical abstractions. Good abstractions are hard to come by; there seems to be an inverse relationship between simplicity and ease, so the plain but powerful lambda is an indispensable tool to the programmer.
These slides loosely follow the tutorial by Raul Rojas and are viewable here.