Kalk (Lime) is a dry Calcium hydroxide or Calcium oxide powder. Kalkwasser (Limewater) is Kalk dissolved in water.
The advantage is that it can maintain Calcium and Alkalinity levels relatively cheaply.
Some soft corals can consume even more Calcium and Carbonate than many hard corals so maintaining Calcium and Carbonate (ALK) is very important for all types of saltwater tanks.
Have you seen the thread titled "Informational Threads" at the top of the Reef Forum?