Elements not typically found in package.xml files can be set (or
overridden) using methods or a config JSON file.
Yes, some of this capability is present in Composer itself, given
Composer's ability to install PEAR packages. However, Composer relies on
PEAR channels, meaning using Composer's classes for this capability would
require developers supporting PEAR and Composer to first publish their
packages to their channel and then generate composer.json files
afterwards.
Using this class, and in particular, the package2composer script that
leverages this class, developers are able to use the package.xml file
they're using with PEAR's package command to generate a composer.json,
which allows a one-stage release workflow to be scripted.