One of the most significant aspects of 1917 is that the actors they cast who are very well-known (Colin Firth, Andrew Scott, Benedict Cumberbatch, Mark Strong) play only minor roles in the film. All four play officers who each appear in a single scene and serve a single purpose in the film. The main characters, Blake and Schofield, are played by actors who are not as famous and familiar to casual movie-goers; yet, it is these two characters we spend most of our time with throughout the movie.
I think this choice of casting not only helps the audience connect better to the main two characters without being distracted by some of the big-name actors, but it also sheds some light on how we sometimes view history.
When we look back on history, specifically military history, we often focus on the men of high stature, position, and leadership, such as officers, country leaders, etc. When I was in school, I learned the names and backgrounds of presidents, generals, prime ministers, and other top officials. When I studied the Civil War, I would only think of Grant, Lee, Longstreet, Stuart, and Chamberlain, famous names of that war.
We often forget about the unknown soldiers, except when associating a casualty number to a particular battle, soldiers who do not make it to our school textbooks yet suffered through the horrors of war.
Because of this reason, it was powerful for 1917 to focus on Blake and Schofield, and cast lesser known actors to play them. We do not know who they are; they are complete strangers to us. Yet we see the story through their eyes, not the officers'. It's one of the many reasons I love this movie.