According to Dan Morill, one of the folks on the Android development team at Google, GPS requires solving a four dimensional set of linear equations – three dimensions in space, one dimension in time. The three dimensions in space are, you guessed it: latitude, longitude...and altitude. A barometer provides a "reasonable first-cut estimate for altitude...which gives you a leg up and can help speed [GPS] lock-on in general," according to Morill.
So there you have it – while the barometer on the Galaxy Nexus could be used to measure atmospheric pressure, that's not its intended use. And the Galaxy Nexus isn't the first Android device to use a barometer for this function. Says Morill, "The Xoom has a barometer too, so it's not a new thing."