Android: Most custom ROMs don't come with Google apps pre-installed for legal reasons. Open GApps helps you install them on a new ROM, and now it can also turn on Google Assistant for you.
Getting Google Assistant on a non-Pixel phone is a relatively straightforward affair already, but it's a little inconvenient. If you're using a custom ROM, you can now select an optional toggle that will modify your build.prop file for you. It's not the easiest method if you want to keep the stock ROM that came on your phone, but if you're already using a custom ROM, it's a convenient new feature.