Appendices store information pertinent to a topic, but not required to understand the topic. They may include examples of code if you're reading a book about C++ or SQL. Or they may include a timeline of an event to provide background to a biography. You may also use the appendices to include charts and graphics, and refer readers to the appendix with that information rather than including it in the core document.
You generally create appendices as you write. You may find, for example, that if you're publishing the result of a study, you want to provide the raw data. In this case, the results are critical, so they form the main body of the document. The raw data may prove distracting if included in the main body, so you place this information in an appendix.
An appendix provides supporting information for statements you make in your core document. If you're writing a legal opinion, you may cite a legal ruling to support your argument. In this case, you may place the entire legal ruling in an appendix and refer the reader to the exact phrasing that supports your case.
Some scholars think of an index and bibliography as an appendix. Though they are appended to the end of a document, they have become so common that many writers prefer to think of these items as separate items of value in and of themselves. Appendices, on the other hand, are regarded as support documents solely.
Some appendices are produced to correct errors in an original version of a document. If this is the case, you must refer to the appendix of corrections to be sure you have the current and correct information. Additionally, if a work is initially published and becomes controversial, the author may write an appendix for later editions that outlines arguments in support of the original work or provides further detail.