What is the gain by using Puppet instead of Augeas itself?

Augeas is a language for describing how to a change a structured file.

Puppet is a framework / language for describing configuration of a computer system. It can store values from other systems, collect them, manage the placement of files, backup old files as they are modified, interact with package managers, and many other things. Altering the contents of files with Augueas is just one of the functions of Puppet.

Puppet's type reference will give you an idea of the different built-in command options that are available.

The gain is in all the other Puppet stuff you can do. If you just want to frob a couple of files, go ahead. But puppet would take care of:

• running that Augeas script everywhere
• making sure the changes are re-applied periodically if needed
• install the augeas packages
• install other packages
• and so on

Augeas is a tool for frobbing a file, puppet is a tool for managing machine configurations across your network. Apples to oranges.