A heart rate monitor can help show your fitness levels, enabling you to calculate a suitable intensity of exercise.
GPS enables global positioning, useful in map, geo-tagging or navigation apps.
Your blood oxygen level is a measurement how much oxygen is reaching your muscles. It is important because low levels mean that you will become easily fatigued during exercise. The more exercise you do, the better your blood oxygen levels will become.
An accelerometer is a sensor used to measure the linear acceleration of a device. A common application is detecting when a device changes between vertical and horizontal positions.
A compass is useful for gaming, maps, and navigation software.
This measures barometric air pressure. It can predict weather changes, for example a sudden drop in air pressure could mean a storm is coming. When calibrated correctly it can be used to determine altitude, which helps GPS devices to lock on quicker and with greater accuracy.