Why did they bury the Terracotta Warriors?

This phenomenon reflects some of the Chinese funeral custom and strict class differentiation in the past. To ordinary people, tombs were like piles of soil with some plants on it for recognising purpose. However, to those rich and powerful, who want to show their superiority, tombs were enlarged to the size of small hills, well decorated and occupied certain amount of lands for this specific use. Moreover, the size of tombs were closely relate to the social status of the tomb's owner, in other Words, even if somebody can afford to build a bigger tomb, it isn't allowed and accepted by the contemporary social institutions. The Terracotta Army guarding the emperor's tomb is an attribute of imperial power so it also of high class status. However, it still cannot be compared with the emperor's tomb, which is a hundreds-meters-high mountain with buildings buried in it.