Understanding the usefulness of different sources is important for historians. It is these sources that help them to formulate arguments and provide accurate depictions of the past. There is many ways a source can be viewed as useful. The date of the source is important. If you are writing about the Battle of the Somme and you have a first hand account from 1916 and an oral depiction of the event from 1945, the one from 1916 may offer a better description of the event. It is also important to understand who wrote the source and why. If it is a commander to his troops describing the war so far (such as General Haig's positive description of the Somme), it must be noted that there are limits to the truthfulness of that source. Nonetheless, that does not make them any less important - they offer something different to the historian. They show how the Generals wanted to convey a certain message. Therefore, there are many things that can make a good primary source. But it is important to look at its date, author, and the author's motives - this will enable you to understand its limitations and its usefulness.