The thing that the letters reveal about why people fought in the Crusades was that Crusades which were a sequence of religious conflicts between Muslims and Christians started with the intention of taking control of places of worship that were sacred to both religions.
Why did people participate in the Crusades?
After centuries of Muslim expansionist warfare, Christians in western Europe organized the Crusades. Their main goals were to halt the growth of Muslim governments, recover the Holy Land for Christianity in the Middle East, and retake areas that had previously belonged to Christians.
"The Franks and the Muslims are done for," is the passage's textual proof. The terrain has been completely destroyed by both sides. On both sides, lives and property are devastated. This issue has been resolved. Jerusalem and the Holy Cross and these areas are all that need to be discussed. Even if there were only one of us left, we would never abandon Jerusalem as the focus of our worship."