What are the uses of the gyroscope in a submarine?

In US submarines, the ships internal navigation system (SINS) uses gyroscopes. The gyros in the SINS keep track of the location of the submarine by following course changes (detected by the gyros) and time and speed on a given course. Crunch all the numbers after entering your starting coordinates, and the SINS will tell you where you are at any given time. And if you happen to get a rock solid fix by whatever means, you can compare it to what the system says. You can also "reset" the system based on your new data.