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
See edit, we've been around for years now and have had many people ask questions when they're new to the site, naturally. We created that primer page to address many of the reoccurring questions.
It also allows you to mass-scan comments when you're (read: I'm) too lazy to pick out an interesting new post and start the conversation on it: