This is needed because ZBD Gamertags are akin to social media handles, they are swappable. In the scenario where a Developer is using a ZBD Gamertag as the identifier for transaction limits and/or thresholds (for example), a user could simply change their ZBD Gamertag and try to bypass those constraints. Using a truly unique and immutable ID like the User ID will provide Developers with the needed assurances.