Modularity
The degree to which a system is composed of parts that can be understood, changed and replaced independently.
Modularity is the property; coupling and cohesion are how you measure it. High cohesion inside modules and low coupling between them is a modular system.
The reason it matters is economic. In a modular system the cost of a change is proportional to the size of the change. In a non-modular one it is proportional to the size of the system, because every change requires understanding the whole thing to be sure of not breaking it. That is the curve that kills long-lived codebases.
Modularity is not free and it is not always right: it costs indirection, interfaces and coordination. The question is whether the parts will genuinely change independently. If they never do, the module boundary is pure overhead.