--Al-Qaeda was born out of discontent with the Saudi government. People are more willing to adopt violence when they don't have access to democracy. Converting a brutal dictatorship into a democracy represents a partial solution to that problem, and provides a positive example to the rest of the region. For this purpose, Iraq was merely the most convenient target--the US didn't trade with them, no alliances would have to be broken, the country lacked an effective military, and despite token outrage, none of Iraq's neighbors were going to miss Saddam after he was removed from power.
--Continued long-term enforcement of the economic sanctions against Iraq was unsustainable--it forced the country into severe economic hardship, and required an ongoing US military presence. The hardship only helped to fill the ranks of religious militants. On the other hand, immediately lifting sanctions would have only enriched Saddam's regime, representing a long-term threat to peace and stability in the region, given the fact that Saddam had attacked three of his neighbors already. The long term ramifications of Saddam and, eventually, his sons continuing to rule Iraq also represented a potential humanitarian disaster.
--Making an example out of Iraq served to intimidate other rogue states. Shortly after the invasion. Libya abandoned their WMD programs, renounced their sponsorship of terrorism during the 1980's, paid compensation to the families of the victims of their terror attacks, and moved to normalize relations with the West.
--It was uncertain whether or not Iraq still had WMD. It turns out they didn't, but Saddam's regime was exactly the kind of regime that could be expected to misuse them if they still had them. This would have been even more of a concern had the sanctions been lifted.
http://en.wikipedia.org/wiki/Rationale_for_the_Iraq_War#Oil_...
"In April 2001, President George W. Bush's Cabinet agreed to use military intervention in Iraq because it was considered a destabilizing influence to the flow of oil to international markets from the Middle East."