Really, the Jewel isn’t so much a battery pack as it is a Lightning cable with a battery bulge in the middle. That 450mAh battery is only good for a ~15-20% charge on most iPhones, but that can be the difference between finishing the night with a working smartphone or an inert slab of glass. It could also work as an emergency power reserve for lower-draw devices like AirPods or a Magic Mouse.
When it’s plugged into a charger or computer, the Jewel also acts as a standard Lightning cable, except that while it’s refueling your phone or syncing data, it’s also recharging its built-in battery. Considering $10 isn’t all that expensive for a regular Lighting cable (hell, Apple charges $19 for theirs), this is a hell of a deal.