The answer is:
To reclaim the Holy Land from the muslims.
In 1095 Pope Urban II adressed a speech to all the catholics to war agaisnt muslims in order to gain power over the Holy Land, this was an inmportant point for both religous and political issues, as well as an important trading point, it was in the way of the spices trade route, and it was the place where catholic church was founded and where Jesus inhabitated, if the Catholic church had power over it they could have power over eastern empires as well.