Raisins, being from grapes, also add some grape sugars, a little bit of tannins (not that you need that with Elderberries!), nutrients unique to grapes, and that quality of vinosity (wine-y-ness) also known as body and mouthfeel. They also tend to add a sherry-like flavor which can be good or can be not as desired. In elderberry, they are a nice combination.