The Schloss Bellevue has only been used by Germany's President since 1994. During the divided years, the President worked at the Hammerschmidt Villa in Bonn (the former capitol of West Germany). The palace was originally used as a hunting lodge by the Prussian royalty. It suffered quite a bit of damage during WWII, but luckily was preserved.
Today the President of Germany has his offices here. He also hosts state dinners and other events for foreign dignitaries onsite. Each elite visitor signs a special page in the guest book. The President of Slovenia had just visited a few days before I was there.