To enhance regional stability. Put simply, the two main motivations for terrorism were the American presence in the Islamic Holy Land (Saudi Arabia), and the Palestinian issue. For the former, the main reason for the American presence was the continued instability that a persistently belligerent Saddam Hussein maintained in the region. Theoretically, if you remove Hussein the entire region becomes more stable and peaceful (I say "theoretically", not because I don't believe it's true, but because I don't believe it's proven, and won't be for another decade or so). Also, if you resolve partially or fully the Palestinian issue, the region should become more stable (and note that Israel intends to pull out of Gaza, settlements and all - and intends strongly enough that they're actually worried about Jewish terrorists trying to derail the withdrawal). And theoretically, such stability that'll contribute to the weakening of the terrorist anti-Western cause (and in this case I say "theoretically" because I don't buy it - they'll find another excuse).
And like I said, that's put simply. Very. But I don't think it's oversimplifing to the point of incorrectness to say that one of the policy foundations of the GWOT is to address and defuse those issues in the Middle East that contribute philosophically and psychologically to terrorism.