Actually, the hook is there to enable residents to pull large, bulky objects up and into a window at the proper floor. Since most homes in Amsterdam have narrow, steep, often winding staircases that make it difficult to bring large, bulky objects upstairs.
Also, many slim houses in Amsterdam have an attic where goods like spices and tobacco could be stored. The goods were not carried up the narrow stairs but hoisted up from street level with a rope using a special beam. As you noticed, the attic window does not have any glass windows but instead has shutters. Pretty interesting!