Google searches its own collection of information. However, other search engines are metasearch engines which mean that they search numerous search engines at the same time. You cover more territory with just a few clicks. Also since each search engine has its own strengthens and weaknesses, when you use a metasearch engine you find a variety of different sources.
Some metasearch engines:
Try them and see if you find better, more diversified, results than with your present favorite search engine. Do you have another favorite metasearch engine?