Why do war novels use the "every man" device?

"In literature and drama, the term everyman has come to mean an
ordinary individual, with whom the audience or reader is supposed to
be able to identify, and who is often placed in extraordinary
circumstances."
http://en.wikipedia.org/wiki/Everyman

War provides a set of extraordinary circumstances for the common
soldier. War novels often emphasize how ordinary the soldiers are -
they often come from small towns and discuss their girlfriends,
family, and friends back home. However, these ordinary people
encounter extraordinary events and conditions when in war.

Lt. Frederic Henry  in the Ernest Hemingway novel "A Farewell to Arms"
is a good example of an everyman character in a war novel. The
extremes of war go on around him and in many ways he is overwhelmed by
it and attempts not to think to much about it. This is a reaction that
many could relate to- rather than the actions of a hero-soldier who
seems invincible.

"The everyman character, however, is written so that the reader or
audience can imagine themselves in the same situation without having
to possess knowledge, skills, and abilities outside their everyday
experience. Also, such characters react realistically in situations
that are often taken for granted with traditional heroes."
http://en.wikipedia.org/wiki/Everyman

In essence, the everyman character allows the average reader to
identify with the character- despite, perhaps, not having any
experience in war itself. The character is humanized and seems more
vulnerable- drawing the reader into their experiences as if they were
their own.

I hope this helps!
-adiloren-ga

Google Search Strategy
everyman and "war novels"

To help the reader(s) identify with the characters, places or plot
lines. The bset books of course are the ones that make people go
"exatly what I was thinking or I would've done" if you relate
something to your audience, your onto a winner and hence "everyman"

The answer and above comment go right to the point of the question as
it concerns war novels.  It seems worthwhile to add the the concept of
"everyman" in literature has much older roots, as this and many other
sites discuss:
http://www.luminarium.org/medlit/intro.htm