importing raw + jpeg

I've never shot/saved in raw format but am going to give it a try. My camera allows me to save in both raw +jpeg. When I view this in Aperture I see a single image and that I can set either the raw or jpeg version as the master. What does this really mean? If I want to edit the raw file I set the raw as the master, and the same if I want to edit the jpeg - I set that as the master. Is that all it really means?