By following people you get the things they post and share in your feed. It's using people to help tailor your feed.

Take a look at my feed, does it look different that yours?

https://hubski.com/feed?id=thenewgreen

Edit: a lot of people will start out by following tags and then transition to adding people as they get to know certain users etc.

Edit2: if you haven't already, I definitely recommend reading the primer: https://hubski.com/primer