The Internet sites are organized by categories or subjects. The categories are organized into different layers, with the top layer starting out broad and getting progressively narrower with each layer. The user selects a category or subject and goes through the different layers until the item is found. Each layer has links to Internet sites. There is no definitive reason to use one over the other, and is really a matter of personal preference.
Search engines are more flexible and allow the user more control over the searches.
However search engines will sometimes give too much information. Because the list has already been compiled, searching is easier. Proper searching can be difficult, especially for a beginner. Search directories when looking for information that is general in nature. Use search engines when looking for specific items.