How is this useful in any way? by definition it's a subscription for people already using the service in the (few) supported cities. If I use it in Denver, why would I care to have early access in Washington?
But I can see it being useful if they're expanding to a nearby city. Waymo is in Los Angeles already and will expand into San Diego this year. If you live in LA, you might have friends, family, or business in San Diego and visit fairly often. Or maybe a group of people want to ride in one car for a weekend trip there but don't want to spend every moment together.