What is the importance of Stockholm in the novel, We Know You Remember?

Several times throughout Eira's investigation, she travels to Stockholm for work. While riding the train, she feels free and untouchable. Once in the city, she meets with Sofi Nydalen. Later during this same trip, Eira connects with Olof's sister, Ingela. While together, Ingela tells Eira more about her brother and her past. Her revelations complicate how Eira understands Olof's old case, and inspire her to reconsider his archived files.