Technical answer: Hard real-time systems.

Non-technical answer: Handing over your implementation to an average programmer.

Try handing your rust implementation over to a Java programmer. Or even your C implementation, which might be worse because they might think they can hack it, but are likely to introduce nasty and hard to find bugs. With Haskell, once they've learned enough Haskell to be able to understand some of your code, if they modify it and it compiles it's (more) likely to be correct.