Hi Ninpo and welcome to DevMaster.
A computer science degree is an accredited degree that trains you to become a self-sufficient computer programmer, scientist, and researcher. This enables you to research new advanced topics and theories to apply in your work. Often you see this in the games industry as someone inventing a new real time lighting algorithm, or a new way to adopt physics on the GPU, new rendering techniques, and so forth. You don't need to have a background in games development to become one. A comp sci graduate would possess the aptitude to find and learn more about the field. A game dev course is more or less designed to get you up and running with game development. It doesn't train you to think about hardware, performance metrics, researching, algorithms, etc. It's analogous to a giving a man a fish to eat for a day vs teaching a man to fish to eat for life.
Of course, not everyone needs either. All it really takes is dedication to push yourself to success. A comp sci degree is more or less proof that you are capable of doing the aforementioned. You could easily prove yourself with a solid portfolio.