That already exists too, but it's also poor UX because it processes your response slower than a person would, and it sometimes misunderstands what you're saying or asks you to repeat.

For the sake of argument, we can imagine the Echo technology is advanced enough to be on par with speaking to a real person, like your own personal assistant that you trust.

I could see it being useful for recurring purchases when I run out of an item, although I (and I assume most people) get these items from the grocery store. For items you've never purchased before, I could also see it being useful if, like a personal assistant, it prepares a short list of items to choose from based on your purchasing preferences. But I would still rather see the list of the items before making the purchase.