Well, Brain, above, gives us one reason. I think Cicero gives us a better one:
Nor, while he is acquainted with the divine order of nature, would I have him ignorant of human affairs. He should understand the civil law, which is needed daily in practice in the courts of law. What is more disgraceful than to attempt to plead in legal and civil disputes when ignorant of the statutes and the civil law? He should also be acquainted with the history of the events of past ages, particularly, of course, of our state, but also of imperial nations and famous kings; here our task has been lightened by the labour of our friend Atticus, who has comprised in one book the record of seven hundred years, keeping the chronology definite and omitting no important event. To be ignorant of what occurred before you were born is to remain always a child. For what is the worth of human life, unless it is woven into the life of our ancestors by the records of history?