I think Apple planned (still is?) to become a carrier as part of their eSIM rollout, but the carriers who sell a lot of their phones are not so welcoming to carrying a competing carriers products.
If Apple can move iDevice sales to their own stores to a higher degree this is more likely to happen.
Click 'Yes' and you're subscribed, billed to your Apple account just like any other in-app subscription, and only used when your main provider has no service or you have no SIM inserted.
Carriers don't want that.