Primary sources are the building blocks historians use to construct their own interpretation of their topic. Different historians would use different blocks, and would arrange them in a different way. Even if the subject matter is the same, every historian strives to create something that is unique, perhaps finding blocks that other historians had missed, rearranging the blocks or turning the previous construction upside down.
Primary sources come in many formats. The same source can be available as a manuscript at the Library of Congress, published as part of a book, in a microfilm set, and also available digitally on the web. Think instead about who created the source, what point of view it represents, how you would gain access to it, and what can you learn from it.
Primary sources are called "raw history' because they are not filtered. Individuals can review these sources and make their own judgments. Primary sources also provide a sense of the immediacy of the historical event or issue, providing insight into the ideas, opinions, emotions and actions that shape history.