We're familiar with the image of an iceberg. Only 10% is visible and the rest is out-of-sight.
If the web were an iceberg, Google searches the visible web and library resources are out-of-sight.
Plus, Google may find unreliable sources. Library resources including GALILEO are more likely to be reliable, credible, high quality & authoritative.
CNN has a nice graphic explaining the deep web and what Google and other search engines reveal vs. what is available in the deep web including library resources.