<<Why was Saddam keeping so much practicly useless but highly valuable stuff laying in a warehouse?>>
Saddam undoubtedly wanted Iraq and the Ba'ath Arab Socialist Party to play a much bigger role in the region, preferably nuclear armed, and believed that the yellowcake would come in handy one day when Iraq would be allowed to do so. There's no question in my mind that Saddam believed that as a sovereign state, Iraq had every right in the world to develop nuclear weapons, same as the U.S.A., China, Israel and a few others.
I think Saddam recognized that for a variety of reasons, some legitimate and due to his own conduct, and others illegitimate and meant only to accommodate Israel's and America's drive for hegemony in the region, Iraq was at least temporarily in the dog-house and would not be permitted to process the yellowcake at that time. However, he had the wisdom to understand that such conditions are never permanent and that one day he'd either be let out of the dog-house or bust out by his own power. A good politician knows that situations are fluid, not static, and he was holding the yellowcake against the day when the current would sweep him and his country into a more favourable position.