I wondered why it had one too and theorised with many people on the exacts on what would make having this so special however none of the ideas we covered were as simple as this one shared by Google Employee and Android Guy Morrill
I’ve seen a lot of hay made about the barometer in Galaxy Nexus. Here’s the skinny; it’s not really as dramatic or weird as people think.
The primary purpose of the barometer is (at least, I’ve been told) to make GPS lockons faster. Locking on to a GPS involves numerically solving a 4-dimensional set of linear equations — 3 dimensions in space, and time. (Yes, you get accurate time for free if you lock on to GPS.) Because of the way GPS works, this can take a few minutes.
This goes much faster if you already have an estimate of your location. This is why "aGPS" (assisted GPS) services are so popular: by starting with a rough city-level coordinate fix through something like cell-tower network location, you can reduce the amount of math you have to do to lock on. This is where the barometer comes in.
The 3 dimensions in space are latitude, longitude… and altitude. The barometer gives you a reasonable first-cut estimate for altitude. This gives you a bit of a leg up on one of the dimensions — especially combined with "2D" aGPS — which can help speed up lock-on in general.
Now of course, the barometer can also be used for things like, well, determining atmospheric pressure (although I’m not sure it’s really weather grade.) But the main reason it’s in your phone is to help with GPS.
Also, it’s worth noting that Xoom has a barometer too, so it’s not a new thing.
So there it is folks, it’s a simple case of locking onto to GPS signals quicker.