Much like the term implies promotions are like advertisements. They are intended to be treated like widgets, the main difference is that they have time scopes. This means you can put together promotional materials and content and set their publish date and time, as well as a finished at date and time to have the promotion disappear. This makes it very easy to schedule launches of campaigns etc.
You can set these on any theme files. We recommend you leave them in the theme files and simply change the content and dates when you need to.