The Wikipedia article is a fair starting point;
http://en.wikipedia.org/wiki/Designated_marksmanIn short though, most armies train for and carry weapons designed to work at 300 to 500 metres, which has been the most common battlefield engagement range since WW2. On occasion though they face opponents who can engage them beyond this range. While the squad automatic weapon can make them keep their heads down, its not usually enough to take them out. Having a man armed with a longer ranged weapon, as well as the skill to use it effectively, increases the 'reach' of the squad in effect. Depending how good he is and the weapon used, this might be over 1,000 metres in some cases.
Where the 7.62mm NATO round is being used, mud and breeze block walls aren't much protection either.
Most people think of them as 'snipers' but strictly speaking they aren't. They are typically the most accurate shooters in their squad, just as the grenadier would be the guy with the best skill set for using the GL, or the gunner with the SAW.