Indexing archives is very important for your consumer base. Since you have an archive to help consumers find information you want to make it as easy as possible. Indexing archives means you will determine how you want the consumer to find the information and then get set up into that format. For example many companies indexing archives on their website break things down by category topic and the month it was created.

A blog, when indexing archives will be broken down by the year it was created and then the month. It will also appear in the topic index. If the blog is about current business news and you have another on the better business bureau you would use the indexing archives to separate them by category.