Having a general understanding of the data and where it comes from is of vital importance for any QlikView application.
QlikView is nothing without data, and without at least a reasonable understanding of the data needed, you will be missing a vital part of the development process. The data may come from several sources, and an overall understanding of these sources is also important. However, detailed knowledge isn't required—just enough to know when something feels right or wrong. If you happen to have detailed knowledge of the source systems, that's good news, but don't get too hung up on trying to learn every detail as you'll never get to start work on your QlikView application.