The battery allows you to bring a weight-saving device; your phone.

It can - within reason - replace maps, guidebooks, emergency satellite beacons, a camera, a secondary flashlight, etc.

You can, if you want, go out with your pockets stuffed with high calorie emergency rations and no pack at all. The weight savings will be tremendous, but at a certain point the tradeoff for weight over comfort and utility becomes too silly.