When instructional objectives have been made explicit, it is easy for anyone to see whether they have been met. For example, if my instructional objective says simply that the student will learn how to write a paragraph, it is hard to assess whether the student has met the objective.
On the other hand, if I say that the student will independently write a 4-6 sentence paragraph that:
-is focused on one idea
The purpose of instructional objectives (for both students and teachers) is determined.