Sniper Turrets are a high damage turret that has fallen out of use with most top players. While they do put out a high amount of damage, they have a very slow firing rate and turning speed. Unable to focus on and easily overwhelmed by fleas and grubs, few players use Sniper turrets. But they can still be useful id partnered with Machine Guns or Zappers. They have the highest damage of all turrets. Also armour piercing.
They "can" become useful for blasting the large enemies. They are very useful on Restaurant, where most of the new enemies have relatively low health, fit for Snipers to shoot down. They can easily shoot down Goo Bugs before they fire their goop (if fully upgraded).
Snipers pose a more interesting option with Termite's turrets. You can use up an important turret slot with a Sniper instead, then use Termite's turrets as the counters.