Theoretical studies have always been a very important method in scientific research. One important reason is that, it is an inexpensive activity. In the past , it was enough for people to have pen and paper to do theoretical studies. Though in modern time, it has been indispensable for scientists to read a lot of journals, publications, and, recently, use the internet to obtain a huge amount of information, theoretical study is still relatively a much cheaper scientific activity. Past study has provided quite a good description of the world and current theory study is providing more powerful tools for us to deal with the seemingly more complex world, such as neural networks, dynamic theory, wavelet analysis, etc. Generally, theoretical study is used to gain a basic understanding of physical processes. For example, from the study of magnetism, Alfven, through the study of magnetized plasmas, predicted the existence of Alfven waves in space, which was later verified by observation. As we have just discussed, observation is most important for us to have a basic understanding of the space environment. But this is not enough. After we obtain enough data, we still need theoretical study to get the physical understanding of those phenomena.
Similar to the observation, theoretical study also has its own limitations. The environment of space is so complex that many processes are involved in the evolution of the space environment. Though the controlling equations of the space environment are themselves relatively simple, it is almost impossible to derive from these equations the nature of many important processes and their evolution. Modern theoretical study has achieved much progress in the study of linear problems, such as small waves, etc. But for those nonlinear problems, such as shocks, etc, theoretical study is still far from mature.