They write the title and description that you see in one of their listings. This assures fewer dead links and that the site has been looked at for content. This also allows for categorization, and this is the biggest difference between directories and search engines. Categorized web pages, by topic, are very handy for those who like to drill down to a particular topic and see what's there. This takes the search engine out of the loop and gives the searcher control of which sites he looks at.
Directories are more like libraries.
All the books on shelves, in a logical order that everyone intuitively knows. The five top Directories listed below also supply many of the Search Engines with much of their data.