This is definitely just the first step; we've got lots of ideas for additional protections we could add, and are monitoring usage and feedback [1] to inform our roadmap.
What this first version gives you is a way to add a tier of trust to your phone number: your Relay number for untrustworthy partners, and your true phone number for important things. That means that data leaks of untrustworthy services can no longer be linked to the important ones through your phone number. Additionally, if you receive a phishing call to your Relay number, that's an extra red flag that it might not be who they say it is.
But again, there is more to come, so stay tuned.
[1] See also https://connect.mozilla.org/t5/discussions/firefox-relay-pho...