This video shows a more sophisticated bit of ARKit, letting you walk around a room and quickly map out the floorspace. While this app might not widely useful, it does show the tip of a pretty large iceberg.
For example, imagine walking/measuring around your entire house and ending up with a reasonably precise set of up to the minute blueprints. You might feed those into another app and place/rearrange furniture to get a sense of what will fit and what works where. You might use the numbers to order the precise amount of carpeting you need, or take your app to a store to make sure that bookshelf will fit in that corner.
Take a look. ARKit is a big deal.