As most know, the WoW Annual Pass was announced with a Diablo 3 incentive. However, many people who would be incentivized by a free Diablo 3 would rather obtain a Diablo 3 Collector's edition instead. In light of that, they offer that if you apply the CE key, you'll get 4 months off of the 12 month annual pass commitment. Essentially paying for WoW for 8 months and paying for the CE to get all of the goodies.
What that process really does is (assuming that you pay for the 1 month installments) save you the base cost of Diablo 3, credited to your commitment that you made to get into this. Overall, their idea is great especially for people who enjoy Blizzard prodcuts and were already going to be invested in either or both products regardless.