Being a Buddy-Buddy type friend takes a little bit more commitment than an acquaintance, because it takes some effort to participate with the person. One tends to have less buddy-buddy type friends than acquaintances— but that is expected (you can't go out with everyone you know).
Why do people hang out with other people? I believe that most people need to feel like they belong— and having friends that you can spend time with fulfills that need. If you read my post on hangers on, there are many reasons why someone wants to hang out with you. You may not always welcome their company, but the fact remains that they want to be with you.
Much like what constitutes a "Date"— what constitutes as going out must be clarified. I still recall the argument with the rest of my dorm-mates in college when we tried to figure out what made a date a date. "No… it's not considered a date if you eat lunch with them in the cafeteria." Likewise, you cannot consider your friend to be your "bud" if you only see them in one place. You cannot claim to have a buddy-buddy friendship with someone you only see in class. You can claim them if you go somewhere other than where you are always encountering them. The lines can be blurred in some sense— especially if you hang around with the same people in many different places. My dorm-mates can be considered buddy-buddy friends because not only did we see each other in the dorms, we also went out for pizza and to the occasional party together.
I think that much like a date, an invitation needs to be made in order for it to be official. The phrase "Hey, want to go to lunch…" can be made— and if they come along it's official.
What about you… Do you find that you have friends that you hang out with? Are you the one who invites others to come along, or do others ask you? What makes a Buddy-Buddy friend to you?