Modularity refers to the tendency of complex systems (either natural or artificial) to be composed of subunits that are less tightly coupled. This decoupling of subsystems has such profound semantic advantages that it has been discovered both by Evolution and by pragmatic social practices such as government and engineering design. See Modularity.
The division of the body into organs and the division of the mind into functional subsystems is not purely for intellectual convenience. When you start pulling a body apart you find tissue planes and abrupt changes in cell type. When you watch a body take shape during embryogenesis, you find that different organs spring from different developmental pathways. Similarly, neuroscience has shown that certain regions and networks in the brain are associated with particular functions: the amygdla with fear, the hippocampus with memory, and so on.