Bedrockgames
I post in the voice of Christopher Walken
For example, do the 5E D&D core books say that once a DM assigns a DC to an Ability Check, that he actually shares the DC with the player? I'm not sure that they do (however, I could certainly be wrong). So you may have some groups that think the player should always know the DC before making the roll, some groups that think they never should, and some groups that think they should some of the time, with some vague wording in the books about using "what works for your group".
I don't know about this one as I don't play 5E. But what this would seem to allow is for both groups who think the DM should share DCs with players, and for groups who think the DM should not not share them, to play the game. It does seem like a playstyle dividing line, so I can see the utility of leaving it open to discussion. If they weigh in one way or the other, a down side is they are essentially picking a playstyle as well. It may seem like a little thing, but these kinds of playstyle differences were some of the reasons people were leaving D&D to go to other games or earlier editions/retroclones. The problem would be when you have disagreement among players. I personally have never found this to be a big issue, but others might. Generally my groups have let the GM decide on those kinds of house rules decisions for a given campaign, and if players object for any reason that is certainly up for discussion (though I don't think we get as contentious about that stuff as I've seen people in the thread here get).