Identifying things at a distance. Your eyes and a monitor works very differently, with your eyes you can focus on far away objects while a flat monitor is more static in its representation of real life.
Realistic games that does require you to identify stuff at long distance usually chooses to reduce this problem this by giving you a key to push to zoom in, like in Arma or Red Orchestra 2.
It may not feel very realistic to be able to zoom with your characters naked eyes, but it actually gives a much more accurate depiction of real life.
This is kind of funny/weird. ITV has made a documentary about the IRA. But somewhere in editing something must have gone wrong, and the actual battle footage seems to have been replaced with a user made machinima of the same incident in the war sim Arma 2.
The Arma 2 clip starts at 0:36.
Depending on your location you might still be able to see the documentary here(If they have not removed it):