Sniper
The sniper is a long-ranged class with the ability to one-shot any enemy in the game. They are valued for their ability to get key picks without directly engaging the enemy team. In a typical 6v6 format, the sniper is considered an offclass, played only when a situation presents itself, most often when a team is at an uber disadvantage, or holding the last point. The main downfall for teams running a sniper, however, is that during an uber fight snipers are practically useless, and that they have very low direct combat abilities. Snipers primarily target the enemy Demoman and Medic, due to the ability to change the direction of a game through key picks.
6v6
Main Article: Sniper (6v6)
In 6v6, Sniper is considered an offclass, however the Sniper is one of the most common offclasses you'll see in the game. Sniper's rely on map sightlines, and tend to lay behind the front lines, in an attempt to pick off enemy players from afar. Snipers are incredibly weak against close up scouts and roaming soldiers, which is why a team should work to protect their Sniper, should they have one. Most of the times Snipers are used to protect or attack the last point of a map. When pushing a last point a sniper mostly tries to get any pick possible so get a number advantage for their team to move up on. When defending last, most of the time it is used to force a med to pop early, or to get picks to stall a push onto last.
Highlander
Main Article: Sniper (Highlander)
The Sniper is considered a pick class in highlander, used to support his teammates while sitting back away from the battle. The Snipers main job is to kill the combo and the other sniper, while taking out unsuspecting enemies in the process.