While Google is one of the best methods to broadly search what is on the web, it doesn't contain all of the information that is available on the web. Some webpages don't contain the information (called metadata) that search engines look for when crawling webpages. Other information, especially in the case of journal articles, can only be accessed through other databases. You learned more about searching these databases in the "Finding Journal Articles" portion of this tutorial.
The short of the story is...if you rely only on a Google search to find information, you could be missing items that would be vital to your research!