Historically, Designed Instruction Works.
People learn better, faster, and more effectively if the system, environment, material, and other elements are designed, organized, and developed in such a way as to make them optimally effective. Designed instruction is more likely to meet with success in its goals of fostering learning, lead to more knowledgable and effective students, and replicable practices of instruction (as well as replicable results).
Instructional Design & Development is the professional and intentional exploration and building of instruction and/or training through systemic & systematic processes. Instructional Designers focus on the psychology of learning through the lenses of behaviorism, and cognitivism, and take philosophical approaches toward the design of instruction such as a focus on social constructivism, active learning, connectivist networking, developing metacognitive strategies, or problem solving.
Instructional Designers follow the ADDIE Process, which are the general phases of instructional design (Analysis, Design, Development, Implementation, and Evaluation), and often specialize or focus on one phase more than others.
Instructional Designers design and develop the instructional environments, pedagogical frameworks (such as choosing an instructional strategy, interactions with media, technology uses, etc.), emphasis of learning strategies, ordering of content and materials, and other subsequent decisions based on sound research and experience.
Designing and developing sound instruction based on strong research and practiced experience is not something that people can do off of the street. While I firmly believe that anyone can learn, it takes a lot of study, practice, and skill development for the results of a design to fall into the category of sound instruction.