Accelerometers can only detect orientation relative to gravity, gyros can detect it relative to themselves. This is the same thing that Nintendo added to their

Accelerometers can only detect orientation relative to gravity, gyros can detect it relative to themselves. This is the same thing that Nintendo added to their controllers a few months ago. For example I wrote an app that calculates Horsepower based on weight and acceleration, however it's hard to determine if detected acceleration is due to the vehicle or gravity (if the device tips or tilts during the test). By having the gyros available, I can detect and compensate for the error-inducing influence of gravity.