What is one reason to use a custom function in Puppet?

Using a custom function in Puppet is often chosen to keep manifests clean by moving complex logic out of the manifests themselves. Manifests are intended to define the desired state of the system in a relatively straightforward and readable manner. By encapsulating complex logic into a custom function, you can streamline your manifest code, making it easier to understand and maintain.

This clean approach allows for better abstraction, as the function can be reused across different manifests and modules, promoting DRY (Don't Repeat Yourself) principles. It enhances the modularity of the code, making it less error-prone and improving overall manageability. Functions can also provide a clearer context for complex operations that require specific input and output handling which is better handled outside of the manifest logic itself.

In contrast, the other options do not align with best practices in Puppet usage. Making manifests more complex can lead to difficulties in readability and maintenance. Ensuring every manifest runs concurrently does not relate to the typical purpose of custom functions, and increasing compilation time is undesirable; rather, the goal of using functions is often to optimize and improve code efficiency.