Part of it is about establishing the likely outcomes so that you’ll have a baseline for adjusting if the likeliest outcomes don’t happen. Having a clear idea of what outright failure and success would mean helps you calibrate the narrative results when a roll gives you a partial success.
But also, it’s about giving players the info they need to fine tune their actions. Since you, as GM, hold the most consequential version of the world in your head, it’s sometimes difficult for players to develop a reasonable sense of how all the parts will interact when they try something. Telegraphing results cuts away a lot of the guesswork. You say, “Okay, you can try swinging across the gap between buildings, but if you fail, you might get hit by a train,” and the players respond, “Oh! I didn’t really register that the tracks ran through that gap. Let me adjust that plan.” Which is fair: a person in the character’s situation would have that information, and if it wasn’t apparent to the player, that’s partly because the game takes place in social, rather than physical, space.