Your sandbox is a place for you to experiment with wikicode, to keep notes, to draft sections to add to articles, or for any other purpose that helps build the encyclopedia. Although your sandbox will usually be left alone by other editors, you do not have 100% control. It can't be used to host copyright violations or personal attacks on people, for example.
Have you ever added an example image to a page, only to have it removed and receive a warning? Here's a solution. If you want to test some unfamiliar wiki-markup, or practice image placement, head to the Sandbox, where you can test to your heart's delight (or you can create your own sandbox for more demanding tests). Or open your own sandbox by clicking on "Sandbox" on your user menu at the top of the screen.