Significance of the opening scene

I'm not sure if you mean the prologue. In any case, Death introduces himself as a character. naturally he is a busy guy during WW2. It is very significant that we accept Death as a reliable narrator for the story.

The significance of the opening scene is that you are not sure what is happening and you might start thinking maybe this is death and he is trying to tell you the truth about dying and not to fear him because he is nothing bu not fair. Death tries to get you to trust him by talking to you as if he were human.