WorldCat is the world’s largest library catalog, helping you find library materials online. I use it for two reasons. One, when I catalog books - it's a great source to find libraries that have the book I'm currently working on, and it's useful to find data for MARC21 bibliographic record, especially subject headings. Second, for reference questions - members sometimes ask for a book that we don't have and that is out of print so we cannot order it. I check WorldCat to see if there is any library near us that could have it and then I give the library's information to the person.
There is probably much more you can do with it, but these are my experiences. I'm excited to explore it more in the future. :)