Server side includes are an extremely useful feature allowing you to omit data from your pages when designing them, and having the server add this data to the file before it is sent to the user. This saves you retyping the information a number of times for different pages, but more importantly makes editing much easier. Rather than being forced to edit tens or hundreds of files with the same updated data, simply place it in the server side include file and have the server append it to the related files.