One reason that solving systems of partial differential equations is so important is the many applications of PDEs in science and engineering (eg. the heat equation, the wave equation, etc.). Often these use numerical methods. However, most of these applications that I am aware of involve systems of PDEs in only a few variables, often corresponding to physical dimensions and/or time.
What are some important applications of systems of PDEs in many variables, or even when the number of variables is growing as a parameter of the problem?
Take a look at Ian Mitchell's A Toolbox of Level Set Methods: