Comment by Aachen

"Sign in with $Clearinghouse" could bring you to a page that prompts whether you want to share a user ID or the phone number, as required, with that service.

The clearing house verifies you only once, or once a year, instead of every time. If the clearing house were to be a nonprofit, perhaps even set up by Signal themselves to spread costs with similar services, that has to be cheaper.

It also gives users confidence that only a randomized user ID was shared, so it won't be used for cross-service correlation and tracking, if the service didn't actually need your phone number but only some identifier.