Ferdinand and Isabella agreed to fund Columbus’ expedition when no other monarch did. Why did they do this? The Spanish monarchs had just retaken Spain from the Muslims, and refused to trade with them. Ever since Constantinople fell in 1453, trade with the Indies was controlled by the Muslims, and they tacked on their percentage whenever a trade took place.
The Spanish royal couple realized that if Columbus was correct, they would have access to a new trade route that was shorter and safer than any other trade route to the indies. They would more or less replace the Muslims as the new middlemen as far as trade with the Indies was concerned, and the potential riches that would result were astronomical.
Despite concerns from their own advisors, they decided to outfit Columbus with three ships and enough money to send him west in 1492.
Isabella and Ferdinand were concerned that their nearby rival, Portugal would dominate the lucrative trade with the Orient if they did not do something to counter the Portuguese. Portuguese explorer Dias had travelled by sea around the tip of Africa to the Orient, and the Spanish were thus motivated to find another, shorter sea route in order to access that rich section of the world. When Columbus came to them, arguing that he could travel west across the Atlantic and arrive at the Orient, they financed his journey in return for a considerable share of any profits. Obviously, none of them knew they were going to bump into a new continent. Also, because they had completed the reconquest of Spain, Ferdinand and Isabella had resources free to devote to this project and the foresight to understand that trade with Asia was of growing importance to European economies.
As to their agreement, Columbus agreed that he would declare Spain ruling power over any regions he discovered and would share the profits of his adventures with them.
There were several reasons why King Ferdinand and Queen Isabella supported the voyage of Columbus. There was a belief that a shorter route to Asia existed. Spain had a strong interest in trading with Asia. If they could discover a shorter route to Asia, it would give Spain an advantage in trading with Asia.
There also was the possibility the Columbus might discover places that were rich in minerals and resources. According to their agreement with Columbus, Spain would keep most of any minerals that were discovered. Thus, Spain had an opportunity to improve its finances and its economy if Columbus was successful.
The main religion in Spain was the Catholic religion. They had a strong interest in spreading Christianity to any land they controlled. If Columbus would discover new lands, he would claim them for Spain. Then, Spain could spread Christianity to these new lands through the work of missionaries.
The King and Queen of Spain sponsored Columbus because they believed Spain would benefit from his voyages. They also knew they would also benefit personally if he was successful. While there were risks, there were also opportunities for many rewards.