I expect this is Google trying to prevent the Android market from becoming a Samsung monopsony. They're half the market, and everyone else is less than 5% (except Hauwei, which is 6%). Given the profits available, I bet half of these businesses are in danger of being shut down every year.
Another thought is that Apple has a huge advantage in that they make really low-level investments in hardware, especially the camera and security devices. Google would probably be willing to make those investments and license them to the other OEMs, but to really be good at it they'll need to be a hardware company themselves.
Lastly, and I think this is less likely, it's just my inner fanboy talking, but I hope Google makes really good phones and makes them Project Fi exclusives. If they can create an anchor phone that draws people to Google Fi, then Google has more leverage to negotiate lower prices from the wireless companies. Anything that take Verizon down a couple pegs is OK by me.