If you look at alternate terms that refer to the same processes as "instructional design", you'll find:
- Instructional Systems Design (ISD)
- Instructional Systems Design & Development (ISDD)
- Systems Approach to Training (SAT)
Notice that the word systems appears in each term. There's an important and practical reason that reflects the reality of a) designing instruction, and b) treating instruction as a system.
A simple definition of any system is a group of processes (or things) that operates interdependently so that if you remove one of the components, the process no longer can continue to work effectively. An automobile is composed of systems. The engine is a system in the sense that if you remove on part of it (say a cylinder, or the exhaust, etc) it will no longer work properly. It's the same for both designing instruction and instruction itself.
First, the various modes for instructional design are systems in the sense that if you skip steps in the model you lose effectiveness.
Second, the process of instruction itself is a system, where, if you remove a component, once again you lose effectiveness. (for a look at a simple system based model of instruction click here)