For a long time Facebook was friends only - so what you posted was only visible to people you knew. They slowly changed the default to friend of friends and then public and that changed the whole feel.
Nobody in their real mind has more than 10, let's say 15 close-ish friends (with whom to share photos of your unborn baby, for example), as such Facebook was never really about friends, it was more about acquaintances.
Sure, however I want to follow what acquaintances are doing too. It's especially useful if they are in my city and I see that on social media and want to catch up with them.