What is an altitude/pressure sensor used for? Basically, measuring. It provides Altitude data to within 30cm (with oversampling enabled). This sensor makes it easy to interface with most controllers since the outputs are digitized by a high resolution 24-bit ADC and transmitted over I2C.
In addition to that, this product provided here can also supply 12-bit temperature measurements in degrees Celsius. Pressure is resolved through an output of fractions in Pascal, and Altitude is resolved through an output of fractions in meter. Using this device, prototyping becomes easy. Note that you can directly get it working since it already contains all the passive components needed.