AFAIK this is only for builds that are not through certification. Xbox has (had?) a way to preview flight builds that do require this, but otherwise the publisher can get 5x5 redeemable codes for games and DLC on Xbox, no Gamertag or email required.
Are people actually doing this??? I've received plenty of early Xbox codes over the years and I've only ever had to do anything like this once for some weird preview build. Occasionally you get a "tell us your gamertag" but even that's rare and typically only for games with always-on servers.
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