Theorists use simulations to test models and compare results to experiment. Experimentalists need simulations to design their experiments and to correct for inefficiencies and systematic effects in their detectors. Monte Carlo or event type simulations, e.g. the phase space of 100s of particles, each with 3 momenta, emitted from a particle collision can only be sampled, not analytically integrated . By creating perhaps thousands or millions of collisions with our simulator, we sample this phase space and can estimate the cross-section for a particular process.
Platform independence allows for easy sharing of code among collaborators.
Strong networking capabilities that allow for such things as parallel processing on separate machines, simulations run over remote links, etc.