There's a great answer from a couple of days ago that refutes a lot of popularly given reasons for the crusades and how important they really were. Here you go, by u/J-Force

Here is another answer that talks about some of the motivations of the crusaders, by u/WelfOnTheShelf

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Hopefully this will tie you up until someone can break down the motivations and which are thought of as being the most important, as well as how true that really is. :)

I saw this post come up in the weekly digest and thought I would add a few points if it is not too late.

Each Crusade had its own causes but for the First Crusade (1096-1099) there were a few major factors:

1. It was seen in Western Europe as a response to perceived Muslim expansion (both recently in Anatolia, Sicily, and Spain but also longer-term in the Middle East)
2. It was seen by the Papacy as an opportunity for to increase its political influence
3. It was seen by the church at large as a way to refocus devout but warlike Europeans from killing each other and kill 'more useful' people
4. It was seen by the Eastern Roman Empire (the Byzantines) as a method for reconquering lost provinces
5. It was seen by Western Crusaders as a way to absolve themselves of their sins for a worthy cause
6. It was seen by some crusaders as a way to gain wealth and power

These six factors probably cover the main reasons for why it began and why so many went. From the 630s Muslim armies had been taking over areas ruled by Christians and had got as far as Spain in the West, were raiding Sicily in the central Mediterranean, and had, more recently, defeated the Byzantines and overrun their provinces in Anatolia (modern-day Turkey) after the 1071 Battle of Manzikert. So although the Muslim advance in general was not exactly fresh news (the Holy Land had been under Islamic control for about 460 years), it was still seen by man Christians as a shameful thing and the Seljuk Turk advances in the 1070s and 1080s had disrupted pilgrimage routes.

In the meantime the papacy was trying to assert its authority over the church both against political power and local church power. They had formally split with the eastern church in the schism of 1054 and were keen to re-establish both relations and Roman primacy. In addition they were constantly battling with European rulers for who should be in charge. In local areas the Catholic church felt they ought to be appointing bishops but often kings did instead. This is called the investiture controversy. When the Byzantine emperor Alexius I Komnenos (1081–1118) sent some representatives to the Pope in 1095 asking for some Latin mercenaries to help in his reconquest wars against the Turks, the Pope (Urban II at this time) seemed to see this as an opportunity to further his aims of both repairing relations with the eastern orthodox church and increasing the influence of the Papcy in the West. The calling of the crusade was hoped to achieve this.

More long-term, Christians had been fighting each other for hundreds of years and the church was sick of it. As you may know, Jesus was a bit of a pacifist and for the most part Christianity preached peace, but the church was struggling to enforce this. They kept making rules that fighting could only happen on certain days of the week but no-one listened. However, paradoxically, most people were very devout in their faith and often felt guilty about the fact they fought so often. When the Pope preached the crusade he essentially phrased it that they could be absolved of sin by dying on crusade. The crusade was marketed as a military pilgrimage, effectively. Jonathan Riley-Smith, in 1987, authored an important contribution to the study of the First Crusade which essentially showed that the vast majority of people who went on the crusade sold their possessions to monasteries but had no intention of staying in Jerusalem, many came home after the conquest. This seemed to suggest that, a few bad eggs aside, most were not out there for material gain but spiritual.

However, some were. Before Riley-Smith crusading historiography was more dominated by left-wing historians who saw the Crusades as akin to a colonial power grab, and although Riley-Smith seems to be right to have corrrected this for the majority of participants, others clearly were out for self-gain. The main contenders would be Bohemond of the Norman contingent. Normans have a track record of invading places, William the Conqueror had taken the Kingdom of England only 30 years earlier and other Normans had turned against the Byzantines in Southern Italy and carved out their own kingdom there. After the siege of Antioch, Bohemond stayed on as ruler and started to carve out his own kingdom in the area. It would seem that he never had much intention of going to Jerusalem.

So basically it seems that a lot of things just came to a head: church power verses secular power, Western church verses Eastern church, Muslims states verses Christian states, and religious guilt and devotion were all funnelled into the First Crusade when the Byzantines gave the papacy an opportunity to tap into. I don’t think anyone anticipated how it would pan out. The later crusades had their own causes but again, for the motivations of Western participants the Holy City was often the goal.