Even if you are not publishing versions of your project with different content you will still need slight variations in your output for different output formats. For example, some topics may not be appropriate in both CHM and PDF output, and some references in the text might need to be phrased differently in each format.
In addition to small variations in content and included topics for different output formats, you will sometimes want to produce versions of your project with larger differences. For example, different topics, topics in different order in the TOC and so on. In addition to this you might also want to generate these versions with a different "look and feel" or corporate branding. If you want to publish different versions of the project in the same output format, for example different versions of the same CHM, you need to define your own output conditions.