So full disclosure, I don't tend to write or read documentation that is external to the in-IDE documentation. But why is documenting the types helpful? The IDE

So full disclosure, I don't tend to write or read documentation that is external to the in-IDE documentation. But why is documenting the types helpful? The IDE, and for Java/JVM there's a choice of multiple, will tell you. Why waste precious hours of life rewriting what is already documented in the type system? Sure I'd document the why of something but the idea types should have any form of documentation external to an IDE that can parse the language just seems... redundant. Whenever I hop over the fence to dynamic languages sure I have to read documentation and large amounts of it is rubbish that could just be inferred from source but that's why I don't hop over there often.