So you need to get an app, song, film or TV show back that is missing on your device, it could be you have more than one device and want the content on both devices, you downloaded it on one and you want it on the other, assuming you have the same account this is easy.
It could also be that you have new device and want your content that you had before on this one. Perhaps you deleted an app or some content to free up space and now want it back. This is something I do, it is surprising how easy it is to fill up space on an iPad.
Apple make this easy to do if it was something you bought through the iTunes store or App store.