The gate is the back gate of the palace, and was used by palace workers. Women being sent into the palace for selection as concubines also entered the palace through this gate.
It is important to note that the Xuanwu Gate Incident, while sharing a similar name with this gate's original name, did not take place at this gate. The palace coup happened during the Tang dynasty, when the capital was in Chang'an.
References
^ Zhu, Qingzheng. 神武门 [Gate of Divine Might]. The Palace Museum (in Simplified Chinese). Retrieved 5 June 2018.