It’s selling an
additional 150k flagship devices. They are selling the Signature line with Google-certified Android by default and those 150k would buy a Pixel to run GrapheneOS otherwise. For smaller OEMs, selling 150k flagships extra yearly is a pretty sizable increase.
Also, the GrapheneOS user base is really growing rapidly, plus I’ve seen quite some people who want to run GrapheneOS, but don’t want to give money to Google, so they might switch when there is a Motorola alternative.
Outside raw user numbers, I think there may be other motivations for Motorola. First, the way they pitched it, the seem to want to position Motorola + GrapheneOS as a secure alternative for business users (so, they may bring in new users themselves). Plus there may be strategic reasons for them to do that. Google has been tightening the grip on Android. Supporting GrapheneOS as an alternative could also be a message to Google - we have an alternative if you tighten the grip too much. Kinda similar to how Samsung has their own App Store, etc. as a message to Google.