In D+D 5e, there are many effects that enable AC, attack rolls, or damage rolls to be modified after being rolled but before the hit is resolves. Examples are the Shield spell and the "Parry" Battle Master maneuver.
When the opponent makes an attack roll against you, is the DM supposed to roll and say, e.g. "does an 18 hit your AC", or are you supposed to tell the DM your AC in advance and he just tells you if it hit or not? This is relevant because in the first case, you will always know if a Shield spell (for instance) would avoid the attack.
Same thing with damage rolls – do you know how much damage was rolled before you decide whether to use a Parry?