Process simulation is an essential productivity feature for any BPM software. The ability to watch a process running and quickly edit it before its deployment to production significantly simplifies and accelerates development of high quality processes.
Automatic testing of processes is equally important, but it cannot replace quality visualization in simulation. Simulation can be considered as process debugging capability by analogy with regular programming. Of course, it is possible to debug a program or script entirely based on execution of its unit tests. However, it is very handy and helpful when you can visualize running code step by step to quickly highlight problems.
It is also important to have fully featured “step into” capability to stop process on every step and evaluate all its variables and conditions. It implies much more detalization than just a basic animation of process flows.