Sometimes I don't have time to do a full charge. For me it'd be nice to have a charger I can plug in for an hour and actually get some juice out of it. Via USB I'd barely get anything for 1 hr or even half hour. Also if you've noticed if you are using the device while charging via USB, it charges extremely slowly. And for only 20$!
Also for me personally, I feel that USB can hurt the device's port long term from constant use. Not to mention I've had my USB charger ripped out of my phone, thankfully my charger was destroyed, not my phone. With the magnets, it seems virtually impossible to hurt the device. Also the website says there is a chip in the device that will keep it from overcharging.
And finally, this charger can free up the USB so you can plug into your computer or use for other uses. Just a few things that come to mind! My 2c.
Ps- I wanna use those little damn magnets! Lol