They are useful when you need general information on a topic, or an overview of what is available on a topic on the Internet, or when you do not have a clear and distinct idea of the information you require. They are a great place to start looking.
Subject trees are a tool for browsing.
The intention is that you navigate your way down a hierarchy of ever more specific items, until you find what you are after. It has extensive support . It is extensive, interesting and well supported. Probably the best place for Kids to get started. It is also regularly updated.