It’s not just about understanding, it’s about being harder to remember. Can you honestly say you remember what your discriminator was without having to look it up?
And even if so, do you really expect that most people would remember it without having to look it up?
The reality is that most people aren’t going to remember a 4-digit number that they didn’t pick.
I have to add people every week and never had a single person that knew their number. If it wasn’t for the “nearby scan” feature, I imagine a lot of those people would have just said “fuck it,“ because looking up the number is so annoying.
But people are used to remembering a username they chose, especially if they can reuse the same one (or two) in multiple places.