You know exactly what article, book, or other item you need.
The item is not available through BCC.
It is not on Google for free.
As defined by the Interlibrary Loan Code for the United States,
"Interlibrary loan is the process by which a library requests material from, or supplies material to, another library. The purpose of interlibrary loan as defined by this code is to obtain, upon request of a library user, material not available in the user's local library."