Showing rather then tell improves what?

Take this example from Kurt Vonnegut's Slaughterhouse-Five:

“It was a movie about American bombers in World War II and the gallant men who flew them. Seen backwards by Billy, the story went like this: American planes, full of holes and wounded men and corpses took off backwards from an airfield in England. Over France, a few German fighter planes flew at them backwards, sucked bullets and shell fragments from some of the planes and crewmen. They did the same for wrecked American bombers on the ground, and those planes flew up backwards to join the formation.

The formation flew backwards over a German city that was in flames. The bombers opened their bomb bay doors, exerted a miraculous magnetism which shrunk the fires, gathered them into cylindrical steel containers , and lifted the containers into the bellies of the planes. The containers were stored neatly in racks. The Germans below had miraculous devices of their own, which were long steel tubes. They used them to suck more fragments from the crewmen and planes. But there were still a few wounded Americans though and some of the bombers were in bad repair. Over France though, German fighters came up again, made everything and everybody as good as new.

When the bombers got back to their base, the steel cylinders were taken from the racks and shipped back to the United States of America, where factories were operating night and day, dismantling the cylinders, separating the dangerous contents into minerals. Touchingly, it was mainly women who did this work. The minerals were then shipped to specialists in remote areas. It was their business to put them into the ground, to hide them cleverly, so they would never hurt anybody ever again.” 

Notice how nowhere in this passage does Vonnegut ever try to force you to feel any specific way about what's happening. We see things from Billy's perspective, but there are very few phrases in here that suggest you should feel one way or another about it. And yet this description of wartime-in-reverse has a profound effect on the reader because we know how these events play forward; the descriptions of the planes sucking the bombs and their flames back up from the group stirs something in us because we immediately juxtapose it with the forward-moving images of bombers destroying towns and cities. We don't need to be told how those images should stir us; Vonnegut's imagery stands on its own to give us space for our own emotional reactions.

The true power of showing over telling is that it lets readers have a much more personal experience with the material, as they're allowed to react to the events in their own way, without being told how any particular one should make them feel.

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