It's an overview of the process of writing a program, covering program and file system structure, build tools, libraries, testing, and revision control. It's not "simply a reference for useful haskell tools" because it has a normative aspect: it says, "This is the way things should be done." It's the document to read when you understand the language well enough to write something larger than a single file solving a Project Euler problem, and want to know what the best practices are in writing a library or executable that could be—for example—published on Hackage or used in production. And while it's not perfect in its presentation, it actually does a pretty good job of what it sets out to do; it was certainly helpful to me when I was in that situation.