[Chris_Newton]

Purely functional data structures are a means, not an end in themselves.

Programming with immutable values and the more declarative style they support does design out at least one infamous source of bugs: shared mutable state. That has become increasingly important in recent years, for example because modern chips support ever increasing numbers of concurrent threads in hardware and because a lot of the software we write is now part of distributed systems.

That programming style has other advantages in terms of readability and reasoning about code. If you can be confident that a particular name in the code always refers to the same value once it’s been defined, tracing the flow of data through a function or through an entire system often becomes much easier. Having more understandable code naturally tends to improve reliability, among other advantages.

If we adopt that programming style then we still need to represent collections of values and different relationships within our data, but we can’t always use “traditional” data structures to do that any more because many of them rely on mutability, which we have excluded. So we need other ways to represent structured data that are compatible with immutability and declarative style, and that is what purely functional data structures give us.