Apple themselves use it to count the number of flights of stairs you climb. Google uses it in Android phones for faster GPS and for indoor location mapping.

I'm using it to forecast the weather by crowdsourcing billions of live atmospheric pressure measurements at Sunshine (https://thesunshine.co).

Other use cases are for migraine headache prediction/prevention and other health-related issues. The primary and most exciting use case however is weather forecasting!

Edit: Basically, barometers are useful for any altitude-related task, as their error margins for relative altitude changes are very small. Additionally, if you can filter out the altitude data as noise, you're left with atmospheric data, which has immense applications when crowdsourced at scale.