The syllabus is the legal contract between the university and the tuition-paying "customer." In has many components besides a simple calendar of reading pace and graded assignments. It spells out the criteria for grading -- usually in percentages (mid-term test, 20%, final paper, 50%, etc.) -- it announces the accommodations for learning-impaired students (signers for the deaf, note-takers for the dyslexic, etc.) -- it states the penalties for misconduct (plagiarism will be grounds for dismissal, 3 absences will result in a failing grade, etc.) -- and it announces office hours, etc. that the professor is required to honor. It is the document of record both for the student's proof of course credit and for the professor's curriculum vitae, and it serves the professor's application for tenure, reappointment, and/or promotion. For English courses specifically, most syllabi begin with a paragraph explaining the themes of that particular course -- in Freshman Composition courses, for example, the syllabus begins with a statement of the "thematic" thrust of the course in toto.
The purpose of syllabus design is really to outline what the course will cover and when, and what the expectations are for the teacher and the students taking the course. Think of it like a road map for both the instructor and the students. Creating a syllabus forces the instructor to carefully consider exactly what the students must learn to be successful and what the instructor must provide to assist that success. For example, if the course is covering the Harlem Renaissance, the instructor must choose what is critical about that time period as there is so much information available that it must be narrowed and focused.
The constraints of syllabus design are much like an essay. With so much to choose from and so much information available, a thesis statement must focus on what is important and what the essay will present or prove. Constraints in a syllabus force choices as a course cannot cover everything there is to know in English nor does the amount of time in a semester allow for unlimited material. For example, considerations which constrain a syllabus could be whether the course meets every day, how much time is allotted for each class, whether the instructor requires outside work such as online instruction with in class being reserved for questions and discussion, which topics will be covered, when and how students turn in required assignments, or in other words, the choices a teacher makes within the constraints of what is possible. I find creating the syllabus the fun part of preparing to teach as I can design how we reach the goals.