Google's privacy page says it does this to "help you get better results using your voice," and that it only does this after you say "OK Google" to learn the sound of your voice and how you speak certain words and phrases.
Google has a whole host of things I've said saved to its servers. It has when I asked the temperature back on Sept. 2, 2014, for example, and everything I've asked since then. It has a recording of my voice — or my wife's — asking Google Assistant everything — such as playing music, turning off the bedroom lights, getting directions.
Normally, this isn't a big deal. I don't mind if it saves a few commands for the sake of creating a better product. But I know that Google can get a better picture of my entire life the more data it has, so I'm glad I can delete all of these recordings.
Like me, you might not want Google to save this information, or you might want to review all of the commands you've ever spoken. Over the past year, Google has made it a lot easier to see the sorts of information it collects, and gives you better controls over stopping it from gathering some specific data.
You can turn it off completely, but just note that this might affect how well Google responds. You can always turn it back on if you run in to trouble.