Because its build on human trait which is "sharing"
Humans are extremely social creatures, when they see something cool, funny, sad or awesome plus it is relevant to them? they can't resist sharing it to their friends and family!
let's say you sell Chiropractic services, so you need to start showing what problems do you solve? what are the struggles? pain? and what are the target market desires? what keeps them awake at night?
Now all you have to do is convey all of that into a form of images, videos and copywriting to from a connection and relevancy with your target market.
So now you started to build a connection where people can engage with relevant content you're putting on your account, You will start to see your followers is increasing, and so as you engagement.
So basically, You're attracting your target market by showing Relevancy + Value in your content
Secondly, share their fears, discomfort and how your products/services solves that for them and get the desired outcome.
This right here is no rocket science, its how humans operate