It can be both. Saying "Alexa, buy eggs" is a lot quicker and easier than loading up Amazon, finding the eggs you like which will probably be the top result for you, and clicking buy (or even Buy Now). Instead, it already knows your preferences in eggs anyway, so just by telling it, you can impulsively buy the eggs without even stopping what you're doing.

Then they get all that juicy "accidental activation" data on top of that.