This is a great question. Let me give a few perspectives. 

First, as you know the crusades were a fight between Muslims and Christians over the Holy Lands. If we think about it for a moment from a contemporary perspective, this is still going on. There is still fighting over religion or to put it more broadly, the ideology of the West and Islam.

Second, in the fighting, one of the great cities of the world changed hands. In 1453, we see the sack of Constantinople. Now it is  called Istanbul. More importantly, many of the intellectuals found their way over to the continent. Through this we would see the Renaissance and the Protestant Reformation.