It was useful for asking questions that neither you nor the other person would want to put in a public post. For example, someone posted a picture in the bad real estate sub that looked very familiar, and I sent a DM asking “is this X location?”. I might not want people to know I’m familiar with X location, they might not want people to know they’re familiar with X location, but maybe we’re both ok with each other knowing.
But I’m pretty sure its main use was sending hate to people without getting banned.
According to the case website, it looks like it’s only people who own a device made by Google that runs their voice assistant. So, Samsung Android users are not included, but anyone with a Google Home device or a Chromecast is included