It's not about having a single device that can use the speed, it's about having a bunch of devices. Maybe none of the devices can use that speed, but when you add up phones, tablets, desktops, laptops, washer, dryer, microwave, fridge, toaster, whatever all connecting to the internet at the same time, you could get closer to that limit than you could with one device.

However, I do agree that gig is way overkill for most households. Even 100mbs would likely be sufficient for most households.