Using historical documents and evidence isnít always as easy as reading a book. There may be contradictions, inaccuracies and bias which we have to take into account when we try to use the evidence.
Just because a source is historical doesnít mean itís reliable. Many of the histories written by early monks like Bede and Gildas were written several hundred years after the events they describe, while the works of the Roman general Tacitus were obviously written with a Roman bias.
Those people who wrote historical documents usually had a purpose in mind, and most of the time they werenít writing for history - imagine if, in a hundred years, someone unearthed your shopping list and used it to try to understand what your life is like!
If we can understand the weaknesses of each source we use, we can make more use of it, and we can perhaps begin to understand a little more about the past and the people who lived in it.