The thesaurus is not just for looking up related terms and concepts. Using the thesaurus, you do not need to find every alternative for a search term. If you are new to a research area you might not know the right search terms to use. The thesaurus helps you to find the concepts.
APA indexers assign thesaurus terms to each article. They capture what the publication is about, regardless of which terms appear in title or abstract or throughout the article. The thesaurus can also suggest to broaden or narrow your search. Looking at the thesaurus is a good place to begin any search. Remember, APA indexers have tagged each record in the database with the terms from the thesaurus that are most applicable to the content of each document. By using the thesaurus first you are putting their efforts to work for you.
Systematic literature search strategies use a combination of index terms unique to each database and key words and phrases relating to the topic. They are important because not everything is indexed.
Use the PsycINFO thesaurus (on top of the search boxes). It is the best way to ensure that you find all the relevant research on your topic.