By taking a beat up car worth $200, and buying some parts for $400, and fixing it up and selling it for $1500, you just created $900 for yourself. This is growing the pie. The is not a set amount of wealth in the world.

Think about the creation of entirely new industries. Think of how much would be spent on cell phones today were it not for the innovation caused by RIM, Motorola and then Apple. Or cloud computing were it not for Amazon, Dropbox ect. These companies create wealth, by creating value for consumers. Banking is a little more complicated, but they provide capital to allow the aforementioned companies to grow and innovate.