I'm wondering about this. I guess back then when it was new (with HoT) people liked to have something different to show to others - for progress - besides the level that existed prior to this.
But nowadays ... unless you have pretty much maxed all the stuff I guess it is meaningless. Someone with like 100-200 could have done HoT first ... or he started with PoF and rushed Skyscale. You just don't know by looking at that number. And for the early progress a lot of stuff is easy to obtain. Just at the very high numbers it might mean anything. (When for a maxed number you know "he even did grind the HoT stuff to max tier" - which in my personal opinion isn't even really grindy.)
I think there could be other stuff. (Or an option for the player to display other stuff.) I like the world completion star. (I guess not many new players do it ... until they might need for the gift for legendaries much later ... but there are other legendaries that do not need core world completion.)
In fractals the party UI already shows (if i am not mistaken) the agony resistance. (Could display on the normal game screen the agony restistance as well - instead of mastery leve l - while in fractals.
For open world maybe some "badges" (like the PvP one) where can chose one. (Similar to titles but harder to obtain. Like the world completion star something for doing all meta achievements of one story season or expansion.)