A major part of the mission of scouts is to control and dominate, if possible, the "no man's land" between the opposing forces so as to prevent the friendly force from being surprised. Typically this is done with outposts and videttes (mounted pickets) occupying a screening line in front of and on the exposed flanks of the force.

While in many cases opposing pickets would have a "truce" between them (at least when the officers weren't looking) there were other times such as at Vicksburg, Port Hudson, and the trenches in front of Atlanta and Richmond where it was worth a man's life to expose a glimpse of himself.

Likewise, when you're moving forward in a brigade or division line of battle, doctrine calls for you to deploy a portion of the force out infront as skirmishers to confirm where the enemy is, look for weak points, and again, keep the main line of battle from being ambushed or otherwise surprised. At a brigade level, you want at least a battalion out there in order to cover your front; division wise a full regiment or more would be deployed.

While there were units such as Birge's Sharpshooters (later the 66th Illinois), Yates' Sharpshooters, and the two famous regiments of Berdan's who emphasized marksmanship and what would ultimately be termed as "sniping" as early as January and February of 1862, the Confederates didn't get really serious about rifle marksmanship until the spring of 1863 when Cleburne incorporated it into the unit schools in his division. Small detachments of snipers were formed with the super-accurate rifles like the Whitworth and the big, heavy bench-rest match rifles, but these were typically controlled at the army or Corps headquarters and parcelled out to the front as needed to take out the enemy's artillery or other key targets.

Plus, the designation as "sharpshooters" gives the unit a little pride, and something to strive to live up to...

Tom