The 'Copy Source to Translation' workflow step option is useful if you want to create localized versions of your site or app that are close to - but not quite the same as - your source content. For example, you might want to make a version for UK English (en-GB), which is broadly similar to your American site.
To do this, you can create a workflow for your near-source languages, edit the Translation step and select Copy Source to Translation from the translation providers menu.
Strings authorized into this workflow will have the Source string copied into the translation field and moved to the next step in the workflow. For example, you can create an 'Edit' step after the translation step and just review the strings for any localized language or spelling.