The terra cotta soldiers can tell us a great deal about how the Chinese lived. The thing that they can tell us most clearly is about how the Chinese soldiers were equipped during this time. They can tell us this because they are made to duplicate what a real group of soldiers would look like. They have all the right equipment, all the right weapons, etc. We can also tell a lot from the soldiers about the state of art in the society -- about how their potters and metal workers crafted things, for example.
Here is a quote that reflects this:
The documentary value of a group of hyperrealistic sculptures where no detail has been neglected from the uniforms of the warriors, their arms, to even the horses' halters- is enormous. Furthermore, the information to be gleaned from the statues concerning the craft and techniques of potters and bronze-workers is immeasurable.