Separation of interface and implementation. In languages without them you must rely on documentation generators or an IDE to explore a library/packages public interface. Header files provide this separation without the need for fancy tools.
why would be using a "fancy tool" a bad thing? Headers are to be maintained manually, which is lost productivity and invite errors. The "fancy tools" can provide specialized search, formatting of the documentation, internal links, formatted and tested code examples, and of course the interface for free. In modern languages they're even part of the included tools.
I'm having a hard time seeing the downside of the "fancy tools".
