Directories are easy to use.
Other search tools, such as the WWW Virtual Library, are directories without indexes. Directories can help, because they let you browse information on many general subjects. From the information you get, you might be able to decide on a general topic for your assignment. You might even be able to narrow down the topic to something you can manage in your paper or presentation. Indexes can help you find documents that have specific words or phrases that are relevant to your topic, while ignoring documents that are not relevant to it.