Up to this point, the Giver has transferred memories of unpleasantness to Jonas only in small, isolated bits. The people in the Community have no concept of what pain is, and Jonas, through his training, has only been exposed to it in the brief experience of sunburn, and the excruciating but passing trauma of a broken leg. The transfer of the memory of war is the first experience Jonas has of pain in the context of a total phenomenon. War is a huge concept, horrific because it is made up of pain in all its forms (Chapter 15).
By putting the memory of war in a chapter of its own, the author accentuates its importance. In its complexity and trauma, it is the climax of Jonas' training as Receiver, and it has the effect of irrevocably changing the way he looks at everything that comes after it. Because Jonas now truly knows what pain is, he can better appreciate the softer, tender memories of Grandparents and love and family; their preciousness is acutely enhanced in contrast to the horror previously experienced. After Jonas receives the memory of war, he begins to think more deeply and actively about what life in the Community really means.
After Jonas receives the memory of war, he lies to his parents for the first time, and he makes the choice not to take the pills which suppress his sexuality (Chapter 16). He confronts his friends who are thoughtlessly playing a game of war (Chapter 17), and begins to seriously entertain ideas about how things in the Community might be changed.