They Started the Banking System
The Knights Templar were an order of individually poor Knights but they were a very wealthy organization. When the order first started, it was nothing more than a group of poor monks but with the official papal sanction came the ability to operate as a charity. This changed the way that the Knights functioned because they were able to take donations.
As their membership grew so did the wealth of the order. Members were required to take a vow of poverty and therefore donated large amounts of their money to the order. They would do the same with their property. More money would come in through their business dealings. Soon the Knights Templar was known for being a wealthy and trustworthy group of monks who had a vow of poverty.
Being a trustworthy group, nobles took to using Knights as Power of Attorney. While they were off on the Crusades they left their wealth and business interests in the hands of the Order and it would be safeguarded until their return. Despite the reputation of the Knights Templar the majority of the order was non-combatants which meant there were plenty of people to manage the business dealings of the nobles.
Since the original goal of the order was to protect pilgrims, they found way to do this by allowing pilgrims to travel without valuables. A pilgrim could visit a Templar house in their home country and deposit their valuables. They would then receive a paper that told of their holdings which they could bring to any Templar house in the world and make a withdrawal. This allowed the pilgrims to have a safe way to travel with their funds and not have to fear being robbed.