I have no idea why they would do this

I do. For the same reason they are cramming LLMs into every other product:

They need to pretend to their investors that there is an enormous market hungry for more of this, so the investors will not demand their money back.

By turning more users of existing services, into “users of Gemini”; by turning more translate requests, more email messages, more search requests, into a “Gemini usage”; they can with a straight face claim more and ever more usage and demand, propped up by users who never agreed to any of this.