"This machine gun automatically attacks nearby enemies. As it is an inert object, the zombies do not treat it like a prey although they can be attracted by their fire."
A Wasp is a stationary turret. Think of this as a veteran soldier but more tanky and immobile. It’s counted as a non-living object so enemies will not go for it at first if they see another living unit/building. If the infected takes too much damage from it it will target the turret instead of the original target. Unlike other defenses, Wasps can't be infected, and are irreversibly destroyed by Infected.
Wasps are economical alternative to Great Ballista. While they lack AOE, they are capable of dealing with small groups of runners and even most types of Special Infected. They also have lower maintenance (2 instead of 15) and require no colonists to operate them. They can also be placed next to other buildings.
In attack missions you can put some wasps near the sides of the pathway, which the enemy will ignore and that will help decrease the numbers.
Wasps block movement of most units, but Harpies can still jump over them. However, single Wasp is capable of taking down a single harpy before it gets to attack anything. Wasps can also form a third or more row of wall behind 2 rows of normal walls to completely block Harpies. Note that wasps need to be powered to block harpies.
2-3 wasps can be used as cheap low commitment defense for resources just outside walls, such as quarries and farms. This is usually enough to stop small groups of runners coming from villages and edges of the map.
Wasps can be used to push energy grid in areas with low infected density even without an army.
Wasps count as attack towers for achievement purposes.