When we read the necklace it was from Madame Loisel perspective and we got to hear from her because she lost her necklace and we could see how she developed throw the story. If it was from the perspective of Madame Frostier then we wouldn’t get when he said how Madame Loisel lost the necklace and bought a new one. We would have never know that she lost it until the end of the story when Madame Loisel came and told her about her losing it.