Google is indeed the most powerful search engine, but when it comes to searching for specific content, Google may only provide some general results. For example, if you're just looking for a song, Google might find information about the singer, lyrics, or even fan websites—depending on the keywords you enter.
This is why there are still a large number of specialized search engines available online. These search engines allow you to find exactly what you're looking for, and because they focus more narrowly, their results are often more accurate.
E-books and PDF files, audiobooks and music, videos, etc., are probably the most common areas where these specialized search engines can be most useful. Make good use of them!