The definition of a veteran is someone with experience in a particular field, usually for a sustained period of time. You may hear sportsmen and women described as veterans, for example, "the veteran midfielder".
Most of the time, however, veteran refers to somebody who has served in the armed forces, particularly during wartime, eg. "Vietnam veteran", "World War II veteran".
It is difficult to quantify exactly what military veterans have done for us as the reasons why a country goes to war are complicated, and in some instances considered unjust.
However in theory, by serving their country you could say that they have protected the civil liberties and rights of a population.
In World War II soldiers protected citizens from the tyranny of Hitler and the Nazis, not only in our country, but in the countries they were fighting for over the whole of Europe and the rest of the world.
The veterans from both World War I and II were revered and honoured following the end of the war, because if it weren't for their contributions and sacrifices, we may have ended living under a dictatorship!