Basic or classic 3D model uses an FlexSim engine driven mechanism of item flow. This mechanism handles events and statistics.
Process Flow is a tier for a graphical logic structure. It can replace partly or completely the FlexSim engine driven logic and statistics mechanism. If you replace the flow mechanism completely, then a 3D model acts as a decoy of visualization.
Typically Process Flow replaces only partly FlexSim engine mechanism, because basic statistic data is still applied by this system. Process Flow assists logic building.
If you begin to adjust predefined templates into custom code procedures, then you will build logic structures in a way always with source code fragments. Then Process Flow is enhancing your possibilities to build logic structures.
It allows a descriptive logic structure rather than a strict source code oriented formal programming.
Process Flow alone is a visualization of logic structures. It can deliver model results without any 3D animation. You can see it as a simple 2D discrete event simulation then.
The answer to “if you need process flow” is, yes, you need it, because there are many functions and coding in FlexSim which are build in Process Flow. It makes model building easier. But you will achieve anything in FlexSim without Process Flow. It is only so much harder.