1. AQ is a political organization that seeks to create change through terror. (That's what a terrorist is. We forget this. Terrorists aren't people bent on creating panic for no reason. They have ideals and an agenda. Terror is their means to effect change.)
2. They seek to raise the power of Islam to that of the G8.
3. They want to change Islam's story about itself.
4. They seek to eliminate the power of the G8 in the Islamic world.
Myself, I wonder how much of this is because Saudi Arabia is such a Dictatorial state. My understanding is that much backing for AQ comes through Saudi Arabia. Why? To guess, I would say that people inside that kingdom don't have a political voice inside the kingdom. However, by working through a terrorist organization, they gain a political voice that neither the Saudis or the West can silence. If Saudi Arabia was more open, and its people more able to create change inside their own country and more able to affect national policy, would these same backers focus their energies more internally, rather than externally?
The important question: what's the payoff? What do both AQ and its backers get if they succeed? Political, cultureal, and financial power. For them, Terrorism pays off if it produces those results.