What is a shotgun for?

This question may seem odd, but I genuinely don't know. I live in a third-world shithole where the need for self defense is ever present, but we have very strict gun control, so purchasing any firearm is very difficult and expensive, and thus I need to really be sure about each purchase. Also, hunting is completely outlawed.

I already have my carry pistol, a Sig P365, a Glock 19 which used to be my carry pistol and now it's stored in the safe as a backup, a .22 plinker for... plinking, and an AR-15 that I keep setup and loaded for home defense and range fun.

I might get me a new gun for christmas, and I'm deciding for either a full-size pistol for IDPA competitions and training, or a shotgun. But I honestly don't know what purpose is a shotgun supposed to serve. Any guidance is appreciated.