Leng Zhuxin being reincarnated into the Leng Family was presumably Leng Hua's design.
As for the purpose, it was to return the Tongtian Tower and the Chenbing Sword, which rightfully belonged to her, back to her.
Everyone in the world knew that the Tongtian Tower and the Chenbing Sword were immortal artifacts used by Leng Hua before his ascension.
Chapter 472