Listen to, or read, his speeches, and those of Zawahiri and other commanders. OBL had a beef with US troops being in Saudi Arabia, and with what he believed to be the corruption caused by globalism and western media. Basically, refried Sayyid Qutb. Atacking globalist financiers in NYC was a gesture of defiance, propaganda of deed to recruit more jihadis. It got new volunteers, but not that many, and he also calculated that the US would only respond in a limited way. He was wrong about that.