Why use a database?
Many researchers rely on Google Scholar to find articles. Sometimes this works really well, other times, less so. (The "less so" is what keeps librarians nice and busy!)
Google and Google Scholar are powered by proprietary algorithms. Because the systems are proprietary, it's difficult to know if we are getting a complete picture of what has been published on a given topic. The algorithm is making the choices.
Library databases match your keywords to words in the title, subject headings, and abstracts (summaries) of the articles in the journals indexed by the database. Some databases also look for your keywords in the full text of the article (which is a mixed blessing). Searching databases, you will find things you didn't find in Google Scholar, and sometimes you will find things in Google Scholar that you didn't find in a database.
The takeaway? Don't limit yourself to one tool!