A quick Google Translate of the original verse confirms that the
original also addressed the reader
Maybe I'm totally missing something here but as a german native speaker, I'd doubt that the narrator adresses the reader here. Contextually, the remaining soldiers of the story are still on focus. It's a rhetorical device, bringing the reader from a quite objective point of view a bit more into the scene. So one can say, the narrator somehow speaks to the soldiers/involved protagonists of the story.
What effect does this add to the poem?
At least a minor effect is the emphasis of an ongoing scene (with unknown result for the narrator as well up to this point) that is been observed more actively by the narrator and the reader both, comparable maybe a bit to a war coverage done by a quite active observer. Without the transition from an objective point of view to a more subjective layer, the aspect of tension might get lost a bit.
In general, a further aspect of this rhetorical device can sometimes be an identification or even an "anti-identification" with a protagonist or relevant group of the story. With focus on Fontane's poem, that might be the emphasis, that this kind of war coverage is created by a member of the fractions involved here (the british soldiers). The opposite, i.e. the "anti-identification" emphasis is sometimes observable within columns in news papers for instance, namely when authors want to emphasize their opposition to fascistic tendencies for instance. Within quite objective articles, you can sometimes observe phrases like "You will never prevail!".