Some of the reasons you might want DITA2Go to use output format names other than the default names, or in addition to names of formats you have already defined:
Legacy format names in outputclass attributes
Legacy format names in outputclass attributes
If your DITA document was created from material authored in a non-DITA environment such as FrameMaker, elements might include outputclass attribute values that correspond to the format names from the source environment. Rather than substitute those names in many settings, or suppress the use of those names for output formats, you can map the names to DITA2Go default format names.
Suppose you wish to produce HTML output, and you already have a CSS file with classes defined for class names that are different from the DITA2Go default output format names. Rather than substitute one of your CSS class names in each of the many settings that assign a format name to an element path, you can prepare a list of aliases that match your class names to those format names.
If you wish to produce RTF output and you already have a Word template with styles named differently from the DITA2Go default format names, you can map those style names to the DITA2Go format names.
Another reason to use format names other than the DITA2Go default names might be localization; see §8.9 Localizing output headings, labels, and names.