It was an integral part of the overall war on terrorism, for several reasons.

(1) Deny Saddam Hussein the ability to ever again develop and/or use WMD, or to allow them to either intentionally or inadvertently fall into the hands of those (Al Queda or not) who would use them in terrorist attacks. This has probably been acheived, although there remains the possibility that Saddam's hidden WMD may be in Syria or still in Iraq. Every intelligence service in the world appears to believe that Saddam had WMD, but of course he won't ever have them in the future.

(2) Deny Saddam the ability to ever again support international terrorism. Although high level connections with Al Queda have not been established, and probably didn't exist, please keep in mind that Al Queda is far from being the only terrorist organization in the world. Saddam's regime has been tied to several other terrorist groups - who will now have to look elsewhere for funding and support.

(3) There are some who believe that the main purpose of the war was in fact for strategic and psychological reasons, to intimidate other countries in the region against harboring Al Queda terrorists, now or in the future. Creating a progressive, pro-Western state with a heavy US military presence (probably have a few bases there for a decade, even after 90 percent of the troops come home) will definitely achieve this goal. There's already evidence that Iran is cracking down on Al Queda members who fled to Iran when chased out of Afghanistan.

(4) A new pro-Western Iraq puts Israel a little more at ease by removing a constant threat (Saddam had already launched completely unprovoked attacks against Israel in 1991 and was still paying large cash rewards to the families of suicide bombers). It helps the Israelis take one more step towards a peace settlement with the Palestinians, who are now influenced by their Iraqi brethern in a much more positive direction. And peace between the Israelis and the Palestinians, with the creation of a Palestinian state, definitely hurts Al Queda and their recruiting, no question about it.

Note that even in the highly unlikely case that WMD are never found, reasons (2), (3), and (4) still serve as more than adequate justifications for improving the Middle-east neighborhood, enhancing US national securty, and strengthening the US led war on international terrorism by getting rid of Saddam.