The Instructional Design process applies the objectives-instruction-assessment approach to teaching and is organized around helping students learn what is worthwhile for them.
Deciding what students should learn is the first major step in instructional design. The goal is to come up with the skills and attitudes that we want the students to acquire. These skills and attitudes are called learning objectives.
Test items are the best instrument to measure how well students learned what you intended (learning objectives), as well as how effective instruction was.