I know how to use Google Scholar, so why would I use a database?
A database often has articles and information that are for a specific subject. Examples include EconLit (Economics), Sociological Abstracts (Sociology), Education Full Text (Education), PsycINFO (Psychology). Plus, databases often have more detailed search interfaces that let you be more precise in your searching. An example is limiting to scholarly, peer reviewed articles.
Google Scholar is a helpful research tool, but it searches many subjects at once. Consider using it along with a subject specific database.