iPhone/iPad gyroscope

Question

I never really understand the applications of the gyroscope on the iPhone/iPad, does it serve the similar function as the accelerometer but like an improvement to the accelerome

Answer 1

"An accelerometer is a direct measurement of orientation, while a gyro is a measurement of the time rate of change of orientation." (1)
By combing the output of the two sensors, called sensor fusion, one can determine the orientation of the device precisely and fast. If you only use accelerometer with a low-pass filter, you still get a reasonable estimate for the orientation but it will lag.

Here is an excellent live demo of both (Google Tech Talk), starting at 21:50.