In general the effort for resolving a problem increases exponentially with the number of parameters you take into account. This implies that if a certain degree of complexity is reached it is wiser to test your solution in a virtual environment rather than trying to resolve the problem through an analytic approach. The three main reasons that can motivate the realisation of a simulation model are :
The goal of those models is to show the problematics, behaviors and results in a visual manner, in order to facilitate the understanding of a layman in those applications. The dynamic flow simulation makes it possible to say, based on concrete results that, for example, this scenario will increase the output by 30% and therefore allows the project to go on.
The task of a dynamic flow simulation is to transcribe the laws and the behaviors of a problematic in a virtual modelisation. Once this virtual model is done, it works as a kind of sophisticated calculator which allows to compute the impact of different parameters on the result you want to increase. For those applications the dynamic flow simulation is a tool which allows to find the best solution. The more complex a problem is the more important, and in some cases even inevitable, the dynamic flow simulation becomes.
For some problems the solution is expensive and therefore the financial impact of potential technical problems is also high. In those cases a dynamic flow simulation is done in order to validate the different technical choices before making the solution come true.
Those 3 reasons are often the main motivation to create a simulation model, but only once you have seen such a model you realise the main power of such a modelisation : simplicity. No other approach is more intuitive than the dynamic flow simulation. If you create such a modelisation you will start by making your solution come true, analyse it and then improve it in order to make it the best solution.