Purely functional data structures are very common in purely functional languages like Haskell but are also used in non functional languages via libraries like i

Purely functional data structures are very common in purely functional languages like Haskell but are also used in non functional languages via libraries like immutable.js. At a high level, immutability forces you to be extremely deliberate about state changes in your application. This improves reasoning/understanding, reduces bugs, and eases debugging. An example of immutability that you might be familiar with would be react props/state. You don’t modify your state. This makes reasoning about state much more simple.