Long ago, the only way to determine the monarch of a kingdom was the "King" or "Queen" nobility title on the kingdom page.
Then, the color of their name on the kingdom page changed to denote the monarch.
Then, the (M) popped up next to their name to denote the monarch.
Then the monarch bonuses went away, making the "King" and "Queen" title officially irrelevant.
So if we have two ways to recognize the monarch of a kingdom, color and (M), plus nothing statistically that differentiates the "King" or "Queen" title from any other nobility title, why still have it?
It'd be nice to both a) be able to show off your achieved nobility rank as the monarch, and b) see the other monarch's rank, just like any other province, so we know what kinds of honor bonuses they are getting.
The Steward has an (S), but is still shows their regular nobility title, so why not let the Monarch have their (M) as well as a regular nobility title as well?