Unless there is an unusually rigid set of controls and policies in the instructor's department, it is generally always the case that the instructor can change the syllabus on the fly. With any decent instructor, there is usually a good reason for doing so, such as incorporating an improvement the instructor has figured out partway through or adding extra emphasis on a section that people seem to be having trouble with. With a poor instructor, on the other hand, I don't think that the situation can be rescued by making the syllabus more rigid.
As such, my interpretation of the disclaimer is that it's simply a way to insulate the professor from complaints of students who take a rigid interpretation. From the students' side, I think it means little, only acknowledging a reality of instruction.