Robots deal in millions of web sites. Directories deal in thousands, or at most, tens of thousands of websites. Robots collect everything and anything, the junk as well as the jewels of internet information. The directories strive to collect mostly jewels, although some are more discriminating than others. In fact, you'll find some directories that are totally indiscriminate and evaluate a web site no more than a robot does.
Some robots are more discriminating than other robots.
The thing you have to keep in mind when using these search tools is this. If you are looking for a subject that is of broad and general interest, like computer software, you should focus your search first on using directories. If you are looking for something that is rare or esoteric, like the postage stamps of New Zealand in the 1920's, you should start your search with the robots first. Some of the search engines perform multiple searches simultaneously on more than one other search engine. They integrate the results from various search engines by eliminating the duplicate results returned.