Digital vs Analog Simulation

While a purely digital simulation (Model-in-the-Loop) is great for testing logic, an analog simulation (Hardware-in-the-Loop) tests the electrical reality of your system. You use values like Pressure directly from your atmosphere model in a digital simulation. In reality, that pressure goes through a sensor which outputs voltage/current. Your electronics have to read that analog signal and convert it to digital before feeding it to your controller. The signal chain:

A real controller output has to "drive" a load. Analog simulation ensures the controller's transistors don't overheat or drop voltage when trying to move a high-pressure valve.

Your internal  Analog-to-Digital Converter (ADC) might add extra error. For example, your atmosphere model says 101.325kPa, but your ADC might convert it to 101.328 kPa due to its internal tolerance. Analog simulation reveals whether your control algorithm is robust enough to handle that 0.003 kPa error without oscillating. It also verifies that your controller’s ADC is actually calibrated correctly. The signal chain:

You cannot "short a wire to ground" in a purely digital simulation and see the smoke. With hardware like NI PXI Fault Insertion Units, you can physically short an analog input to a 24V rail. This allows you to verify that your hardware's protection diodes work and that your software enters a "Safe State" immediately.