Why did princess de take charge of the phoenix seal for the queen?

Because Emperor Sarila, whom Kangxi loved deeply, was the only queen in Kangxi's life, but she died in childbirth when she gave birth to Ermei. After her death, Kangxi read that she had been reluctant to establish a queen for their common sake, and that Princess De was kind and virtuous before Kangxi. In order to prevent the harem from being ownerless, Kangxi put the imperial concubine in charge of the phoenix seal, but it was never established again, so the imperial concubine became a princess for life, but later her son Yin Zhen, the fourth elder brother, became the emperor, that is, Yong Zhengdi.