For me, it is certainly filled by my cellphone. But I can see the method to their madness.

An Amazon Echo isn't a needed device. It just makes certain actions easier (e.g. playing music, checking the weather, listening to the news, etc), all of which can be satisfied with a cellphone and computer too. But the key is, it makes these actions EASIER.

For someone who's already indoctrinated to an Amazon Echo device, I can see the barrier for purchase being lower than someone who doesn't have one. For such a customer, the appeal again is that it makes even more actions EASIER.

Being able to see your baby monitor, make a phone call, check the news, see who's at your front door (integration with doorbell video services coming up, I'll bet!), etc, are all achievable by other means. But this family of devices makes them all EASIER.

I think that's the key here. And granted, these devices aren't going to appeal to everyone. But for those that already have the gateway drug/device that is an Amazon Echo, I can see them investing further into this ecosystem of devices.

My use case is walking around the house, remembering we need something at the store, telling Alexa about it, and then looking at the Alexa app when I get to the store. I don't walk around the house with my phone in my hand.

Yes, with a teeny bit of extra convenience. These things are a cellphone on the bookshelf that you don't have to hold or recharge. Also, bigger screen and better speaker.