Why is he so important to Florence?
The Medici family was extremely rich and full of power, which even allowed them to take over some of the city government. Cosimo was, during that time, the most powerful man of politics in Florence. He was actually as powerful as a king because he had the money, the responsibility, the respect from people and the power, though he didn't want to be known as the king of Florence, even though he was capable of doing so. He also brought peace to Florence by equally exchanging precious gifts and putting a lot of money into the City to make it a better place...
...He traded different precious items, for money or other riches, to get the best he could for his City. Cosimo also built the most famous and beautiful churches in the City so that all of his sins would be forgiven.