There are two basic ways to get I/O, build it or buy it. Which path you took was fairly straightforward before Soft-I/O. We think that the equation has changed with Soft-I/O.
Build your own I/O system
Let's say that you are designing a automobile engine controller for one of the car companies. You are going to build more than a million of this design over a period of time. Your design is pretty fixed in terms of how many inputs and outputs you'll need. The electrical interfaces for the sensors and actuators will be pretty well defined. The fast I/O, such as the fuel injectors, will probably hook up to the main engine controller board. Slower I/O, such as the turn signals, might connect via a bus like CAN bus to reduce wiring. The point is, however, that you will have an I/O list and you will design some circuit boards that are intended to connect together to form your automobile engine control computer. The cost of this system, if you amortize your not-insignificant development costs over a million units, will be low. The flexibility of your design will be poor. That is, if someone decides to add a water injection system timed with the fuel injectors, you will probably have to make a hardware revision to your board.
The build your own I/O system involves selecting various integrated circuits with the correct voltage and current specifications and adequate precision and noise immunity. You then lay out a circuit board and then build a prototype and test it extensively. An example circuit board is shown above. In the end, the design is entirely yours. It is a proprietary design to you only. Proprietary can be good or bad, but we'll cover that later.
A significant advantage of building your own I/O system is that it connects to your system exactly the way you want it to connect. If you need three wires to go to a sensor, then you provide a connector with three pins, and you connect up your sensor. This advantage will become clear when we contrast it to the purchased solution.
Now, let's move through some gray areas and look at the other alternative, buying your I/O system.
Buy your I/O system
For this situation, let's imagine that you are a machine builder making forming presses for the automobile industry. You are going to build a few dozen presses per year and you are going to sell them to the automobile plants where hoods and doors are formed from sheets of steel. Your design is not too tightly fixed because your presses have various options. One might have a part ejector and another might have to interface to a third-party ejector. The conventional way to handle the I/O for your press would be to purchase a Programmable Logic Controller (PLC or PAC). An example PLC is shown to the right.
A PLC is a building block kind of product that's usually made up of a backplane, CPU, power supply and then I/O modules. Most PLC's will have dozens of different I/O modules. Some PLC's will have more than 100 I/O modules. There are modules for digital inputs, analog inputs, digital outputs, analog outputs and then special functions such as counting or controlling. Each class of module--for example digital input--is made up of a number of sub-categories covering the type of input circuitry.
One result of this modular approach is that connecting your sensors and actuators is not straightforward. For a variety of reasons, the PLC cannot provide for a simple connection to its modules. Connecting a three wire sensor might take seven wires, and you might have to add other components, such as additional power supplies.
The attraction to you to buy the I/O system in this low volume case is that the development cost is considerably lower. Designing and then testing a circuit board design is costly and time consuming. In addition, your customer, the automobile company, wants an I/O system that they understand and work with. They certainly do not want a proprietary system: they want something that they can maintain themselves.
The Soft-I/O Option
Until recently, there was no Soft-I/O option. But, there is today! 
Let's give you our--admittedly biased--view of how Soft-I/O compares with other I/O approaches. We believe that Soft-I/O gives you the benefits of the build it yourself while providing you and your customer the benefit of the purchased I/O.
Because you purchase Soft-I/O, there is no development cost to amortize. Thus, for anything but very high volume products, Soft-I/O is the lowest cost option.
What makes Soft-I/O different from either other approaches to I/O systems is configurability. Recall that the build it yourself path got you a board that was perfectly configured for your application (as long as it does not change)! The PLC, on the other hand, had various modules that you could purchase, but the modules have fixed configuration, so your effort is to configure your sensors and actuators to match the fixed configuration of the PLC.
Because Soft-I/O is configurable, a single part number Soft-I/O module can configure itself to virtually any sensor or actuator. The result is an I/O system that is as flexible as a build it yourself board but as supportable as a PLC.
Let's give you our--admittedly biased--view of how Soft-I/O compares with other I/O approaches. We believe that Soft-I/O gives you the benefits of the build it yourself while providing you and your customer the benefit of the purchased I/O.