human behaviour and the environment can't be modelled. Real traffic is bursty. Analytical tools have to parametrize noise into unrealistic distributions like Gaussian, Poisson in order to get an answer. Real data has lots of hidden dependencies which is hard to model. An earthquake might make everybody in a geographic area use the internet at the same time. That kind of cross correlation is hard to model correctly. The only sure way of knowing a system behaves well in a realistic situation, is to run it. Real data can answer questions like how big the caches need to be etc.