Why did people go on the First Crusade?

The First Crusade, which occurred between 1095 and 1099, was the first attempt at recapturing the Holy Land. The military operation was initiated on the executive orders of Pope Urban II during a speech at the Council of Clermont.
The operation was to aid Byzantine Emperor Alexios I Komnenos. Prior to the First Crusade, the Byzantine Empire had lost a large portion of Anatolia (present-day Turkey) to the Seljuq Turks. The First Crusade was both a military conquest (or re-conquest) of the Holy Land and a political message meant to show power against the Islamic expansion.
The First Crusade was preceded by an unsuccessful attempt at recapturing the Holy Land called the "people crusade," which was made up of peasants and common-folk and was initiated by Peter the Hermit. They attacked Jews in the Rhineland on their way to Anatolia. The group was annihilated by the Turks.
https://www.britannica.com/topic/First-Crusade