Notably Discord stopped using that format two years ago, moving to globally unique usernames.
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...
It was honestly a downgrade i ended up just putting the 4 digits I had before at the end of my username cause surprise the name was taken immediately
The stated reason is obviously not able to justify the change; either they have an internal reason they're not willing to admit to, or someone at Discord just went crazy.
Imagine trying to connect with your friends... by telephone.
The actual reason here, implied but not stated outright in that one, is that Discord being a public platform, having only numbers to discriminate between users makes it extra-trivial to impersonate someone else. Obviously you can still do some of this with unique usernames (you see slight misspellings, adding harder-to-see characters like periods, etc, as strategies), but these are more complex to execute on at scale and easier to block once and reduce the impact, vs being able to use ~arbitrarily many post-username numbers.