Functional programming only starts to show its power with types and parametric polymorphism (higher kinds, etc...). Without type annotations, all you get is a watered down, crippled version of what a functional language can achieve.
Second... While I love static typing as much as the next guy, there's definitely value in immutable data in a dynamic language. You also get Dialyzer in Elixir/Erlang, which is an interesting alternative to the usual type systems you find in Haskell/Rust/etc. in that it focusses on finding places where there's provably a problem, rather than where it's not provably correct.
Sure, but there's even more value in immutable data in a statically typed language, because of all the advantages that come with such a language (automated refactoring, better performances, tooling, etc...).
I should know better than to use the word "pure" on the internet, someone will always correct how inaccurate my usage is :)