Encyclopedias help you figure out the who, what & where of a topic. Knowing the who, what & where of a topic is the first step in researching a topic and discovering the "how" and "why".They can provide background information and can be used to identify key people, events and dates which can then be used as search terms for finding additional information. Many also provide a jumpstart on your research by listing important books in the bibliography at the end of the entry.
So you're going to ask grandma what she did during the summer of love. It's going to be helpful to know a bit background info. Sure the wikipedia article is a good start but you probably should know a bit more about the period.