There are two things it does do, John.
1 – it forms one of the filters on what you want to see on the main wall (Activity tab on main menu). You can toggle through the sub menu and choose to only see what friends have said. Right now we’re a small forum of 70 but if we hit a couple of 1000 of active users, you might very well prefer to just see what friends have posted! 😉
2 – if you ever create a group, you can invite friends to join in. Not sure if you can invite non-friends although they can certainly request to join.
Otherwise no it doesn’t make much difference but we’re a clannish species and people like to pal up it seems 🙂