In my opinion a definite answer to your question is difficult to give. If we need some integrated or average value, it is possible to use a deterministic model. But if we want to simulate, for example, fluctuations in the number of molecules or molecular trajectory, we must use the stochastic models. Fluctuations in the number of molecules is modeled as a random variable on the basis of the Poisson distribution. The position of a diffusing molecule (Brownian motion) can be described by a system of three stochastic differential equations.