Configuration management is quite a dynamic problem. In other words, the systems that need configuration are mostly moving targets. In some situations, system administrators or operators get lucky and work with large quantities of 100 percent uniform hardware and software. In most cases, however, the landscape of servers and other computing nodes is rather heterogeneous, at least in subtle ways. Even in unified networks, there are likely multiple generations of machines or operating systems, with small or larger differences required for their respective configurations.
For example, a common task for Puppet is to handle the configuration of system monitoring. Your business logic will likely dictate warning thresholds for gauges such as the system load value. However, those thresholds can rarely be static. On a two-processor virtual machine, a system load of 10 represents a crippling overload, while the same value can be absolutely acceptable for a busy DBMS...