The ostensible reason for urban's call to the crusade was to liberate the holy city of Jerusalem. ?

Liberating Christian Jerusalem was also the real reason for the call to Crusade. Abu Hakim Mansour 'The Mad Caliph' out of Shia Fatimid North Africa's destruction and desecration of Christ's tomb at Jerusalem and dismantling of the giant Church of the Holy Sepulcher 1009-1011 is fact not fiction and the main reason there were the Crusades in the 1st place. But Urban also voiced another reason and historians recognize a 3rd.

Urban's 2nd voiced reason was reopening Christian pilgrimage that had been cut off, both by mad Hakims action (that even shocked all Islam) and by the semi barbarian Muslim Seljuk Turks holding Turkey. Pilgrimage back then for Christians and Muslims alike was big business and responsible for 30+% of trade plus a main way to practice religion.

The 3rd reason historians count (but Urban didn't dare voice) is he wanted to get the Christian knights out of Europe. In 1096 there wasn't any main wars. The European knights were only bred and trained to fight and kill but there wasn't anything for them to do at the close of the 11th century. So, they had been stomping around Europe fighting and killing each other but mostly killing the peasants/ the farmers and burning crops and towns. Urban wanted it to stop and sending them all to the Middle East to be what they were trained/ meant to be was a good idea and went along with the 1st 2 reasons.