The Templars essentially introduced banking to Europe. Initially, people noticed the Templars had well fortified and guarded quarters in the major European cities, a reputation for integrity, and operated with the pope's approval and without submission to kings or local nobility. So, the Templars began to take deposits for safe keeping. It was also a great way to keep funds from the levies of local nobles and bishops.
As their wealth increased, Templars began loaning money to anyone they deemed credit worthy.
Since they were taking deposits for merchants in a number of trading centers, they then began to offer a service where a merchant could deposit a sum with the Templars in one city, and pick it up in another. A merchant could send a ship from London to Calais, transport the goods to Paris, sell the goods in Paris, deposit the money with the Templars in Paris, avoid bandits and pirates on the way back to London, and collect his money in London.
This system was very similar to the Islamic Hawala that had been operating in the Mideast and South Asia for a few hundred years, and it is very likely the Templars encountered it in the Holy Land.
Banking became one of the major Templar operations, and actually contributed to their demise when King Philip the Fair of France decided it might be better to destroy the Templars rather than pay what he owed them.
In The Templar Concordat, the power of the present day Templars is anchored in international banking.