What is a rabbet joint used for?

there are different styles for rabbet joints and different reasons for using a rabbet joint. Rabbet joints are a fairly strong wood joining method which is primarily used for joining the two ends of two separate pieces of wood. Your basic rabbet joint is an "L" shape where the inside angle of the "L: from one piece of wood is joined with the outside angle of another "L: edge of another piece of wood. In this case the "L" creates a 90 degree groove-bed for accepting another piece of wood to be joined.
commonly used for bookends