Explain how knowledge of the file structure of libraries enhances online searching?
This is a good question. Let me make a few points to help you here.
First, you need to realize that many libraries are huge. The collection can be unmanageable. For example, Harvard University has more than 16,000,000 books in their collection. In light of this, you will need some powerful and sophisticated search options. Precision is the key word here.
Second, the more you know of how the data is structured in libraries - both electronically as well as physically in the stacks, the better your search experience will be. As you can imagine, you will have to be selective.
Finally, many libraries have all of their resources online - at least their call numbers. In view of this, you can do some very sophisticate searches - key words, by year, publisher, etc. The more you know the online process, the better your research will be.