Equipped with custom-modified sniper rifles, sharpshooters accompany raiding parties as scouts and long-range combatants. A well-trained sharpshooter can pick off an unprepared explorer before they even know there are enemies nearby.
The skill of many of these sharpshooters is, ironically, the result of Initiative colonist defense training before leaving the Milky Way. Strategically, it was believed that well-armed snipers would be simple but effective defenders of their outposts, with a lowered risk of losing personnel in the event of an attack. It appears this training has now been passed along.