beppee, I've never really given it too much thought.
It does make it easier to just click on the person that is on your list of friends on your profile page and send a message.
You can also use the Quick Links to open a dropdown menu and choose "Open Contacts Popup". A window opens that lists all your Contacts (friends) and lets you know if they are online now.
Anyone can ask anyone else to be a "friend" (you don't have to accept or even ask anyone). I've had new members ask me and I've asked more senior members.
Personal belief determines whether you consider that person a true "friend" or not. I have many "friends" in my life outside MR, but they are only called that because I know them better than others. I would classify most of them as an "acquaintance" (Knowledge of a person acquired by a relationship less intimate than friendship.) Others, I classify as true friends, as we have known each other for years and been supportive of each other through the various ups and downs in our respective lives. We can call and/ or come visit whatever the time of the day or night if we need help or support. (plus, I just know that a true friend would help you "hide the dead body". <-- I read that somewhere and loved it.