It limits Google's abilities to force bundling of their revenue-generating and platform-psitioning components.
Giving Android away to OEMs is in large part a vector to get the Play store, Chrome, Maps, and their monetizable services and data scarfing on the device.
But some OEMs don't bundle everything. Think of the de-Googled Android variant ran by the Kindle Fire series, or the Chinese phones that include local stores and service providers that work there instead. Google can try to bludgeon them into playing ball with the trademark and various over-the-top agreements, but they can't stop them from saying "We'll take AOSP and make our own version and call it FireOS."
With full licensing control up and down the stack, they can close that door much tighter.
