TOO many people think WTC is a technical excercize? I've never heard this before. Of course, one is too many...
I think the main benefits of fugues like these are, as said, contrapuntal thinking, linking the concepts of counterpoint and harmony, and perhaps in general a subtler understanding of music. WTC is truly Bach at his most sublime.
Daevren - my counterpoint professor at university would also like to add that WTC is Bach's attempt to popularize the (then revolutionary) concept of equal temperement (''well-tempered", "Wohltemperierte") by writing a unique prelude and fugue in every possible key, including the exotic ones like F# which could not musically exist in older tuning systems. Of course, his examples of keyboard-tuning were so persuasive that they remain among the most absorbing works now three hundred years later, and ~35 Hz higher.