Answer:
To keep people from finding out where their real grave is. In fact, Cao Cao's tomb was not originally a secret, but no one knew the location of his tomb since the Song Dynasty. Coupled with the great influence of the Romance of The Three Kingdoms, it was widely believed that Cao Cao built seventy-two tomb tombs before his death.
Cao Cao was a famous strategist and statesman in The Three Kingdoms Period. In order to maintain the normal life of his army in the turbulent society at that time, Cao Cao created the posts of Commander Faqiu and Lieutenant Grojin in the army. He specially stole ancient tombs to supplement his military pay. Chen Lin, one of the "Seven Sons of Jian 'an", wrote a call to arms "for Yuan Shao has considered Yuzhou", which recorded Cao Cao's various charges. In it, he recorded: "Cao Cao also specially placed a lieutenant named Faqiu and a lieutenant touching Jin. It also confirmed Cao Cao's behavior of robbing tombs.
Cao Cao tomb robbery theft, seen after the tomb was stolen everywhere grave, understand the reason that thick burial will attract thieves. So in the arrangement of their own affairs put forward to "thin burial" requirements. He wrote in the "Final Order" before his death: "Ximen Leopard Temple on the west of the original for the Shouling, because of the high base, not sealed not tree." Before dying, he made it clear that he should be buried in ordinary clothes, that he was not allowed to put jewelry in his coffin, and that he would be buried west of Yecheng.
Cao Cao's sons Cao PI and Cao Zhi both wrote about their father being laid to rest, and Jin literati Lu Ji and Lu Yun also wrote that Cao Cao's coffin was escorted by generals Sima Yi and Jia Kui. All these contents are recorded in historical books, which are groundless and reasonable. The claim of "72 suspected tombs" is not credible.
Because Cao Cao was buried simply, his tomb began to deteriorate within a few years, and tomb raiders did not take it seriously because there were no valuable gold and jade funerary articles. Cao Cao's tomb was not built a cemetery, nor was it sealed with trees, so it quickly deteriorated. Several hundred years later, the tomb disappeared without a trace. In the Tang Dynasty, Li Shiming even made an offering for Cao Cao's tomb, and in the Song Dynasty, Cao Cao's tomb was only recorded in a historical book. Coupled with the popularity of the Romance of The Three Kingdoms, which cast Cao Cao as a villain, the story of Cao Cao building seventy-two suspected tombs in order not to be known where his real resting place was passed down to this day.