The way I see it, immutability and purity are hard requirements for actually enforcing the contract, and a very robust type system is then a requirement to go from code that makes you want to shoot yourself in the head, to code that's incredibly expressive and easy to reason about. Haskell just happens to sit in that exact niche.
I agree this is important. I have just started to move a project to Java from Python and see that immutability and purity are attainable for certain parts of the system but appear to cost a lot (leaning hard on use of interfaces, for example) in terms of readability.
