On Google's [specs page for the new Nexus 5](http://www.google.co.uk/nexus/5/), the Nexus 5's built-in sensors are listed as:

> 
> **Sensors**
> 
> 
> GPS  
> 
>  Gyroscope  
> 
>  Accelerometer  
> 
>  Compass  
> 
>  Proximity/Ambient Light  
> 
>  Pressure  
> 
>  Hall  
> 
> 
> 
> 

(Click "View Full Tech Specs" to see that list)

Looking up [Hall Sensor on Wikipedia](http://en.wikipedia.org/wiki/Hall_effect_sensor), it seems to be some sort of magnetic sensor, but it's not the compass because that's listed separately, and none of the example uses on the page seem to make much sense for a smartphone application.

What is it, and what is it useful for?