This approach goes a long way toward democratizing the name space, since nobody can "own" the tag prefix. (10000 people can all share it). This can also be used to prevent squatting and reuse attacks - just burn the full account name if the corresponding user account is ever shut down. And it prevents early users from being able to snap up all the good names.
Their stated reason[1] for doing so being:
> This lets you have the same username as someone else as long as you have different discriminators or different case letters. However, this also means you have to remember a set of 4-digit numbers and account for case sensitivity to connect with your friends.
[1]: https://support.discord.com/hc/en-us/articles/12620128861463...
For buckets I thought easy to use names was a key feature in most cases. Otherwise why not assign randomly generated single use names? But now that they're adding a namespace that incorporates the account name - an unwieldy numeric ID - I don't understand.
In the case of buckets isn't it better to use your own domain anyway?