Quentin Skinner on why studying the historical context surrounding a written text is to already be engaged in interpreting that text, and to be doing so in a better way than by studying only the text:
It enables us to characterise what their authors were doing in writing them. We can begin to see not merely what arguments they were presenting, but also what questions they were addressing and trying to answer, and how far they were accepting and endorsing, or questioning and repudiating, or perhaps even polemically ignoring, the prevailing assumptions and conventions of political debate. We cannot expect to attain this level of understanding if we only study the text themselves. In order to see them as answers to specific questions, we need to know something about the society in which they were written. And in order to recognize the exact direction and force of their arguments, we need to have some appreciation of the general political vocabulary of the age. Yet we clearly need to gain access to this level of understanding if we are to interpret the classic texts convincingly. For to understand what questions a writer is addressing, and what he is doing with the concepts available to him, is equivalently to understand some of his basic intentions in writing, and is thus to elicit what exactly he may have meant by what he said - or failed to say. When we attempt in this way to locate a text within its appropriate context, we are not merely providing historical 'background' for our interpretation; we are already engaged in the act of interpretation itself (pp. xiii-xiv).
Skinner, Q., 1978, The foundations of modern political thought, 1st ed. Cambridge University Press, Cambridge.