The ultimate goal of a simulation is to take a model, perform a calculation and explain/predict some phenomena. The goal of a theory is to make a model to explain/predict phenomena. The simulation requires a model to work on, the theory is the entrust to provide that model.
The widespread misconception about theory and simulation may be due to the lack of traditional laboratory work. Since both theoretical and simulation work can be done from a desk in front of a piece of paper and a computer, from a bystander’s perspective, they appear identical.