Historians are experts in their fields. A historian whose field is, say, Euro-African relations on the West Coast of Africa in the seventeenth century will know exactly who all the major players are, why they are there, where they came from, their motivations in being there, their relations with each other, etc. That information is called context. Context is used to make sense of primary source documents.
Primary source documents are often one-sided, but because the historians studying the document in question understand the context from which it came, they are able to productively analyze and ask questions of the document. This allows them to read between the lines of the document and strip away the document’s bias to see what the document is actually saying, or, what the document really means within its context.
Primary source documents are the lifeblood of the historical discipline. Without them history as a discipline would not exist. The majority of primary sources are one-sided, thus, if historians were to reject the historical importance of one-sided documents, our understanding and knowledge of the past would be severely limited.