The Sandbox is just an empty page that anyone can use to experiment with wiki markup or page formatting ideas. Although you can always make a change on any page in the main namespace and then immediately undo it (provided no one else has edited the page in the meantime), the Sandbox is intended as a low-pressure space where you can play around without worrying about messing up another page.
It's easy to wipe clean after you're done:
- either go to the history tab and click "undo" next to your edit
- or just click to edit the page and delete all the content.
Individual versions of the page history can always be accessed later. For example, I can look at the story tropes draft by going to the history tab, clicking "last" next to the edit version I want, and then clicking "permanent link" under the Toolbox menu on the sidebar.