It was hypothesized at the time, the radiation might provide a defensive shield above the U.S. against Soviet nukes, but aside from the light show, it ended up frying many of our satellites. The radiation took 10 years to dissipate, which made study of our natural radiation belts, the Van Allen belts, problematic during that period.
Wikipedia has a good article explaining the test and NPR has a good article and video about it from a few years ago:
Back in the summer of 1962, the U.S. blew up a hydrogen bomb in outer space, some 250 miles above the Pacific Ocean. It was a weapons test, but one that created a man-made light show that has never been equaled — and hopefully never will:
