Confusing about create a sub-domain

I have a little confusing about creating a sub-domain. In my mind, to create a sub-domain we just came to domain provider website and create it, then create a record to point the sub-domain to an IP host (this IP may be different than the main domain IP), and this 2 hosts (main domain's host and sub domain's host) can be 2 different web server.

But when I read some guide on the internet, and have a confused that why they create sub-domain in the host provider panel (may be called cPanel), then point it to the directory in host (regular in public_html folder). Is this just for a server to serve some static files purpose? Or I've missed something.

Anyone who knows it please explain to me. Many thanks.