From the Pure Primer:
Learning and practicing the art of functional programming and term rewriting is a refreshing intellectual experience that forces us to develop a totally different coding strategy and to rethink previously learned approaches to algorithms and data structures. In many cases, the functional approach allows us to write shorter programs that work the first time and are easier to maintain. This experience makes us better programmers in any language.
People unfamiliar with functional programming may think that the pure functional paradigm -- programming without side effects and without changeable variables -- is very inconvenient. However, it appears inconvenient largely because it is new and unfamiliar and needs getting used to.