One of the Google engineering team that works on Android is Dan Morrill and he has cleared the air on why a barometer is inside the Galaxy Nexus and for that matter the older Motorola Xoom has a barometer as well. Morrill says that the barometer is there to help the smartphones and tablets get a GPS fix more quickly and accurately. As some of the comments point out a barometer is almost the same as an altimeter.
The barometer helps the GPS device solve a set of equations needed to get a signal lock with more speed and accuracy. The barometer could certainly be used for other things too, but the main reason is to speed GPS locks according to Morrill. Now you know!
There have been some folks wondering why among the sensors that the Nexus has, Samsung chose to include a barometer. After all most of us, tend to think of a barometer as only useful for weather forecasting.