Is that a barometer in your pocket?
The answer might be yes since a number of smart phones contain a small digital barometer. The reason is likely to help determine altitude, or change in altitude, and work with the GPS system. It has potential to create detailed pressure maps and improve forecasts, but the problem would be determining the exact elevation. GPS can only estimate your elevation which could lead to significant pressure errors should you try to create a Sea Level Pressure map with phone pressures. Still it is impressive that what was, and still is, 30 inches in length can fit into your pocket (no jokes please).
“The pressure is 30 inches of Mercury (inHg) and falling” is a common way to express air pressure. A tube was filled with Mercury and then stood upright into a base. The liquid in the tube would lower creating a vacuum but would eventually stop, leaving a measurable gap at the top of the tube. Evangelista Torricelli is credited with creating this device and noted that the height of the mercury in the tube changed on a daily basis. He concluded that the atmosphere itself must have weight and the that is what holds up the mercury, not the force of the vacuum. It’s tough to find a mercury barometer these days given that some countries have restricted the sale and use of Mercury.
Aneroid barometers are much more compact and can be used in boats and aircraft. They measure the pressure by subtle changes in a small metal capsule and how it expands and contracts depending on the surrounding air pressure. All of the calibrated pressure sensors are used to provide valuable data to weather forecasters pilots. The important word is calibrated, whether knowing the devices elevation and reducing the pressure measured to sea level or adjusting a planes altimeter (pressure sensor to determine altitude) to give an accurate reading depending on the pressure at the surface.
Finally now to the digital barometer that is small enough to fit in your phone. Itself can give accurate pressure estimates and could potentially be used for weather forecasting if not for 2 issues that we see initially. One is collecting all that data and two would be determining the accuracy of that reading knowing the exact altitude of the phone, not just a GPS guess. Once these were addressed then you could create very high resolution pressure maps that could be analyzed to create better forecasts. 
