A grenade launcher is a weapon that can fire a grenade more accurately, with higher velocity, and with greater distance than a typical human being could throw it unassisted.
This is a good thing because when it comes to zombies and things that go boom, putting some distance between you and them is always a prudent course of action. This weapon is definitely not for use in close quarter combat. With the explosive power of some grenades you’re sure to take out quite a few of the undead but that pretty much means you’re going to be taken out yourself. In such situations you’re better off with your traditional side arm, an automatic rifle, or (in a pinch) a pair of heavy knives.
Now, imagine yourself however at some distance from a horde of shambling undead. It’s going to be tedious taking them out with your assault rifle and too far away for your shotgun to be able to do some real damage. This is where a grenade launcher comes into play, blasting the main horde then picking out the stragglers with your piece of choice afterwards.
Grenade launchers can either come in the form of standalone weapons (single-shot or repeating) or attachments that are mounted under the barrel of a rifle. The most common grenade round in use by modern militaries is the 40 mm fragmentation grenade, which is effective against a wide range of targets including infantry and lightly armored vehicles.
It’s not for everyday use of course but for defense and extreme situations you’d definitely be glad you brought one along.
Just be careful when you pop your load.