According to legend, it began with Qin Shihuang, and the emperor used Zhu Pi to review the throne of his courtiers. After Qin Shihuang unified the six countries, the number of memorials reported by various places suddenly increased. Qin Shihuang found that when marking the memorials of ministers with black pen, his reply was easily confused with what ministers wrote, and it might also cause the problem of missing approval. So he thought of a way to write instructions on the memorial with a red pen, which was not easy to make mistakes and greatly improved the administrative efficiency. The use of Zhu Pi was also used by the later emperors, which shows that the things formulated by Qin Shihuang had a great influence on later generations from the state system to the bamboo slips. Now when the teacher corrects the students' homework, he also corrects it with red pen. I think it is also influenced by the use of Zhu Pi by ancient emperors!
Since ancient times, emperors have paid attention to uniqueness. They have many special things. For example, they can call themselves? Me? No one else can wear a dragon robe. Whoever surpasses it may be beheaded. Ministers' letters are written in black pens. If the emperor's reply is also written in black pen, it is not easy to distinguish. Therefore, emperors of all dynasties chose to use Zhu Pi, thus distinguishing the difference between monarch and minister.
In a word, the emperor used Zhu Pi mainly to improve work efficiency, make the reply easier for ministers to see, and also to show his position.