What do we mean by configuration data? There will be lots of pieces of information in your manifests which we can regard as configuration data: for example, the values of all your resource attributes. Look at the following example:
package { 'puppet-agent':
ensure => '5.2.0-1xenial',
}The preceding manifest declares that version 5.2.0-1xenial of the puppet-agent package should be installed. But what happens when a new version of Puppet is released? When you want to upgrade to it, you'll have to find this code, possibly deep in multiple levels of directories, and edit it to change the desired version number.
Multiply this by all the packages managed throughout your manifest, and there is there's already a problem. But this is just one piece of data that needs to be maintained, and there are many more: the times of cron jobs, the email addresses for reports to be sent to, the URLs of files to fetch from the web, the parameters for monitoring checks...