Learning the history of the English language, or any language, is important for a variety of reasons. These include a better contextual understanding of unique syntax within the language, social changes and language adaptation throughout history. Understanding the relationship of English with other languages is largely dependent upon its history.
The first resource is the University of Oxford, Faculty of English. The course on History and English is a combined course focusing on history and the English language including text productions during historical time periods. The importance of the connection is the primary focus of the course, but the overview of the course addresses the need for this class.
The second resource is a large class layout provided by the University of Duisburg-Essen in North Rhine-Westphalia, Germany. The class, Studying the History of English provides a large volume of information about English with eleven chapters focusing on different areas. The reason to study English is not as explicitly stated, but clues can be found in Language Change (2) and Language Contact (5).
The last resource is Seven hundred reasons for studying language by Angela Gallagher-Brett as part of the The Higher Education Academy. The work has literally found more than 700 reasons to study language. Providing 700 reasons may be a bit verbose, the opening chapter focuses on the need to study language in general which should shed some light on why it is important to study English.