There is a lot involved, but the basic idea is that your rifle and sight need to be calibrated to match from where you’re shooting to where you’re shooting. The journey to the sniper hides was long, arduous, grueling, and not exactly guaranteed to be hassle free. You might be stumbling over miles of dirt with stray wires, holes, dead bodies, broken equipment, rubble, and so on, at night, in pitch black. Then wandering through narrow trenches packed with people going both ways, again with dead bodies, stray wires, holes, and broken kit. There is a non zero chance that the scope you sighted in yesterday could get tapped or knocked, and it will be out of alignment by the time your arrive at your hide.

The use of debris was a simple way to sight in. You tell your spotter that you’re gonna shoot that tin can just by the sandbag over there. They tell you where you’re hitting so you can adjust your sights. Naive snipers would fall for this trick, but later on experience taught to use puddles instead.