As processors begin to have a plethora of I/O connected to them, they may still have plenty of processing resources available but run out of available pins. At this point, there is a decision to make. ICs meant to provide port expansion are available, but should they be used? If you're designing a system with reuse in mind, it is important to see whether a subsystem approach can be employed, instead of creating a huge monolithic design, where all of the hardware and firmware is intertwined and tightly coupled. Sometimes, when the pin capacities of a single MCU are reached, it is an indication that the MCU is performing the functionality of several different subsystems. Often, if these subsystems are broken down and individually developed, they can be dropped into future products without modification, which can greatly decrease future projects' risks and schedule.