Why use a database in addition to Google Scholar?
Databases limit the material they include to technical literature within a discipline.
They have consistent information, and often additional information than Google Scholar.
You can sort by publication date more accurately than within Google Scholar.
There are generally easy ways to refine your search.
Arrangement of references is in a database, not just harvested from the web. This means logical searching is possible.
Ways in which Google Scholar is Still Necessary, in Addition to Databases
Google Scholar sometimes can search within the fulltext of documents.
Google Scholar is better for interdisciplinary searching, or if you don't exactly know what words to use in a search.
Google Scholar does not require special syntax or connectors.