Content on the surface web is free and open to the public. Some of it is intended to inform, some to advertise, some to persuade; search engines like Google and Bing "crawl" and index this content, at which point it becomes available to you as search results.
Content on the deep web is not freely available. This could be because it is owned by publishers, media outlets and organizations who want to make money by providing access to it. Perhaps the information is confidential, like medical records or private posts on social media. Or it is not in a format that can be easily indexed, like large collections of data from scientific research projects. This content is usually password-protected.
The dark web is used by people who need to hide their online activities for whatever reason. They could be criminals using the internet to pursue illegal activities. They might be dissidents using the Internet to mobilize a protest movement, or terrorists using it to recruit members and coordinate attacks. Sometimes journalists might use the dark web to protect anonymous sources from exposure. This content is encrypted and routed through the Internet in such a way that the sender and receiver of information are anonymized.
You'll notice that "academic databases" are in the list of things that are housed in the Deep Web. That means that when you are searching using Google, for example, you are not seeing the content of those databases in your results. So you could be missing important sources if you only use Google for your searches.
Even Google Scholar, which does provide access to the indexes of some databases such as Science Direct and Web of Science, does not include all academic database content. Also, Google Scholar may show you that an article exists, but then direct you to a website that requires payment in order to display that article.
Using library databases instead can allow you to: