Stingers are one of the beginner weapons, along with rocket and blades. They are stationary. When they make contact with the enemy, it causes continuous damage. Stingers are useful if they make contact with the enemy for a long time as they usually have high damage. Chainsaws and drills are the similar weapons for stinger.
You don't have to unlock it this weapon, you automatically obtain it.
Pairing the stinger with a harpoon can be a great idea as the harpoon locks the enemy near your machine, giving opportunity for stinger to inflict high damage.