When the Galaxy Nexus was first announced by Samsung at AsiaD in Hong Kong, they kind of glossed over the fact there’s a Barometer inside the device. Because they didn’t tell people why it was in there, we all just assumed it was there for the lulz or for developers to make interesting applications wrong. It turns out we were only part right. The Barometer can indeed be used by developers for weather-related applications and what-not, but its main purpose is to help the GPS get a location fix in a shorter amount of time.
As Dan Morril explains, to get a GPS location fix you need to solve a 4-dimensional set of linear equations: latitude, longitude, altitude and time (fun!). If you have a rough estimate of where you are, usually through aGPS, GPS performs less equations and can pick up your location quicker. However, to get a leg up on the equation for altitude, the Galaxy Nexus uses the estimated altitude from the Barometer to help the GPS perform even less equations to get your location resulting in a quicker accurate location fix. Impressive.