of years by accreting stars and gas, and by merging with other black holes.
Spin results from a transfer of angular momentum, like playing on a children's swing. If you kick at random times while you swing, you will never get very high. But if you kick at the beginning of each downswing, you go higher and higher as you add angular momentum.
Studying a supermassive black hole also allows theorists to test Einstein's theory of general relativity in extreme conditions.