Let's just say that, in general, a book's subject matter will determine its call number. So all books about widget technology will be shelved in the same area. One important thing to remember is that in a given library, every book will have a unique call number. No two books will have the exact same call number. In general, the bigger the library is, the longer and more complex the call numbers on the books.
Many people have trouble deciphering call numbers.
If you don't read the call number correctly then you can easily put the book back in the wrong place. For all practical purposes, an incorrectly shelved book is lost. If the book isn't where LEO says it should be, then the next person who needs to read the book can't find it. When you incorrectly shelve a book at home, you only need to eyeball all of the books in your bookcase until you located the stray volume. But libraries might have thousands or even millions of books.