If your friends asked you to meet them at the Starbucks in San Jose, you'd never find them if you didn't know which Starbucks they meant. To find where you need to go, you need an address. It works the same way with items in a library.
The King Library has over 2 million items on 9 floors. How can you find out where you need to go? It's simple: the call number for each item is its address.