From the product page:
Attaching the MagSafe Battery Pack is a snap. Its compact, intuitive design makes on-the-go charging easy. The perfectly aligned magnets keep it attached to your iPhone 12 or iPhone 12 Pro — providing safe and reliable wireless charging. And it automatically charges, so there’s no need to turn it on or off. There’s no interference with your credit cards or key fobs either.
At your desk and need a charge? Just plug a Lightning cable into the MagSafe Battery Pack for up to 15W of wireless charging. Short on time? With a higher than 20W power adapter, you can charge both the MagSafe Battery Pack and your iPhone even faster. And you can track the status of your charge on the Lock Screen.
The pack is $99. A premium price, but an elegant solution if you own an iPhone 12 series and find yourself running out of charge towards the end of the day.
The battery pack has two 1,460mAh cells (according to this tweet).
The total of 2,920mAh is right around the capacity of the iPhone 12/12 Pro, more than the mini, less than the max, but nowhere near the $20 battery packs, like this model from INIU, which sports extra ports and 10,000mAh of charge.
That said, the MagSafe Battery Pack is slim, very tightly integrated (charge shows up in the battery Home Screen widget), uses the same Lightning cable as the iPhone, and, of course, has the convenience of MagSafe. The battery will also pass-through charge (charge when you plug in your phone).
If you own an iPhone 12 series, this will give you a constant trickle charge, topping off your phone, getting you through the day if that’s ever been an issue.
From Apple’s official MagSafe Battery Pack Support Page:
Attach your MagSafe Battery Pack to your iPhone and then plug the MagSafe Battery Pack into a power adapter. Using a 20W or higher power adapter, the MagSafe Battery Pack can charge your iPhone with up to 15W of charging power.
You can also charge both if you attach your MagSafe Battery Pack to your iPhone, then plug your iPhone into a power source. You might want to charge this way if you need to connect your iPhone to another device while charging, like if you’re using wired CarPlay or transferring photos to a Mac.
The benefit of a separate power pack like the INIU mentioned above is the additional power (days of charge without having to recharge the pack) and the flexibility (multiple ports, and with the right cable, you can charge all your devices). But the cost is a bit of extra weight and the need for a cable.