Source:

Shotgun slugs are often completely pointless. No, really. See this? No point. It’s round. Ba-dum-tsch! Thank you, thank you. I’m here all week. In all seriousness, the purpose of shotgun slugs is to make big damn holes in things at ranges longer than where a bunch of smaller projectiles can be reasonably expected to stay clumped together to make a similar big damn hole.

If you fire a load of small balls, there is a chance that some will get deflected. Also, due to having less mass, they aren’t going to go as far, or transfer as much energy to target. A slug is all one piece. All the energy it carries goes into what it hits. And that energy makes a big damn hole. Slugs are the grandchildren of musket balls. They are relatively slow, and use mass to make up for lower velocity, while delivering sizable amounts of energy on impact. Historically, they have been used by hunters because guns were very expensive until mass production was invented, and having separate ones that fired shot for small game and bullets for big game just wasn’t affordable for a lot of people. Sometimes, it was also impractical. Carrying two long guns while out in the wilderness by yourself just doesn’t work very well.

Slugs were (and sometimes still are) also used for defense against dangerous animals. It was common practice in some areas, like remote parts of Siberia, to carry double-barrel guns with birdshot in one barrel for the actual game you were looking for and a slug in the other in case you came across a bear who was a bit atypical and decided to be annoyed with you rather than run away. In the modern day slugs are still used for larger game. There are some hunting zones close to populated areas that only allow shotguns due to them having lower velocities and shorter ranges. Slugs aren’t going to fly as far as rifle bullets, which makes for less chance of having a stray projectile clear the woods and hit someone’s house.