1) me as a player. I tell the DM my Elf walks into the bar. She describes the bar, including a whole adventureing party. I go to buy a room and one thing leads to another and the dwarf NPC from that adventureing party picks a fight (verbal) with my elf. after RPing back and forth the Dwarf tried to intimdate my elf. I am not my elf, the dm is not the dwarf, we are playing those roles though. becuse this GAME has a stat call cha and a skill called inimadate the DM rolls and tells me she got a 4 (roll of 2+2 prof no cha mod). I now make an informed choice knowing that this was not very intimadating. (in some alternate world maybe she rolled a 19+2 for a 21, and I would know they were VERY intimadating) at no point did I loose or forgo agenecy here. I am still controling my elf. I still get to decide how he reacts.
2) me as a DM. two players want to buy a dog from a breeder. I as the DM know I have stats for a cool smart dog better then the MM/PHB that I have been sitting on for months. player 1 and NPC talk, I have him brag about how his dogs have both blink dog and dire wolf in them... player offers 2gp and I have NPC laugh and say "For a rare powerhouse like this, no that will be 10gp." Now the PCs decide they think they can push around the breeder. One aids the other and they say they are "intimadating the breeder into taking the 2gp" I as the DM tell the one with the higher skill to roll with advantage, and they get some huge number (it doesn't matter lets say a 27) now I have no rules in any book or even in my notes on how the breeder reacts. I have to decide quickly. SO I have him fall backwards afried, and the dogs all move up and growl... no rule no roll took away my againcy, and it is the same for the PCs