Apparently, the Crusades were to spread Christianity and to get Jerusalem and other Christian-based cities back. Jerusalem was almost totally inhabited by Muslims at the time.
[quote]The Objectives of the crusades was at first to release the Holy Land, in particular Jerusalem, from the Saracens, but in time was extended to seizing Spain from the Moors, the Slavs and Pagans from eastern Europe, and the islands of the Mediterranean.[/quote]
(Not to mention the people there were also being attacked by Vikings.)
I agree with Agent about everything God does was planned. He may've had the Crusades take place so that Christianity would spread. Maybe He just wanted a lot of people to die.
(http://www.historylearningsite.co.uk/cru1.htm, http://en.wikipedia.org/wiki/Crusades, and http://www.middle-ages.org.uk/the-crusades.htm for references. Quotes taken from the last one.)