Why subsume transcription and textual study within a History of the English Language course? Because close textual study cultivates an understanding of the intimate link between historical language change, the human (i.e. social, political, economic, migratory…) forces responsible for it, and the speakers and writers (dead though they may be) whose texts we use for evidence.
Sometimes historical linguistics excises writers and textual context from study of language change (and sometimes this even makes a degree of sense, e.g. Grimm’s Law). But more often, yoking textual analysis to study of diachronic language change lays the groundwork for deeper, sociohistorical understanding of change, change which occurs neither immediately nor spontaneously, and certainly not independent of the speakers and writers of a language.