Sima Qian was appointed grand historian of the Emperor Wu's court in 108 B.C. In his Records of the Grand Historian he describes the events which he witnessed or heard of that occurred during his lifetime, which offers the modern historian a fairly accurate account of a contemporary historian of the Han Dynasty. He composed his records from conversations he had with courtiers. In addition, he also consulted a plethora of documents and files which were stored in the palace as well as having had the ability to interview generals which enabled him to comment on the military institutions of the Han Dynasty.
Sima Qian had the ability to accompany the emperor on his visits to the provinces where he was able to record the "barbarian" tribes and lands which were brought under Han rule by Emperor Wu. In Sima Qian's records we also have evidence of the penal system and the conditions in the prison system. Qian wrote very detailed descriptions of these penal conditions for he had an intimate experience with them, having been punished for his "attempting `to deceive the emperor'" (xii). He was accused of using "veiled" words in his description of Emperor Wu, which was Qian's way of criticizing the emperor using language and words that were not outright critical, but inferred disapproval of the emperor and his actions.
The purpose of his official history is to record things so that the people who will be reading the history will be able to understand their past. This need to understand where one comes from helps states and individuals determine how and why they got to where they are. He organizes his accounts thematically, he discusses the military, generals, and he offers a geographic and ethnographic account of the peoples in the various "barbarian" provinces as well. He repeats himself often in order to get a certain point across.
The second way in which official histories are used is that they teach people important lessons. As Thucydides articulated in his Peloponnesian Wars Book I:
It will suffice if my work is judged useful by those who wish to gain a precise understanding of past events because, due to human nature, such events, or ones resembling them, will occur again at some time in the future (Thucydides, 1.22.4)
Basically, if you don't understand or study history, and the mistakes that were made in the past, then you are doomed to repeat them. He is very detailed in his descriptions because the more detailed a history is then the more accurate it appears to be by those who read it.
In contrast to Tacitus, the Roman historian who wrote about the Roman frontier, Sima Qian's descriptions of the barbarians occupying the Han frontier lands is more accurate, possibly because he was able to actually visit and see "first-hand" these barbarians whom he writes about, whereas, much of Tacitus' description came from second-hand sources, and usually merely stereotypical.
As an official history of the Han Dynasty, this book serves its purpose. It allows modern historians a semi-accurate look at what life was like along the borders of Han China. As a source, Sima Qian has recorded a history which makes a valiant attempt at staying clear of certain stereotypes which other historians such as Tacitus could not avoid, that of the uncivilized "barbarian". Rather, Qian's description of the frontier peoples is written from the barbarian perspective, therefore more reliable.