There are a few other important UP requirement artifacts in addition to use cases; this chapter introduces them. If you want to skip this chapterwhich deals with the secondary topic of requirements rather than OOA/Dno problem. Jump to the classic OOA subject of domain modeling on p. 131, after first reading the summary of iteration-1 requirements starting on p. 124.
So why include this chapter if it isn't central to learning OOA/D? Because it gives cohesion to the case studies and offers a more complete requirements example.