I agree with all the reasons above for a friends list. Personally, I can't remember names and match them with people easily, so I use the friends list to help me remember which username a person I've spoken with has. I also often have a lot of trouble just being social with people, so I'm not good at inviting people to be friends either. However, sometimes I try to force myself, just so I can work on that skill.

I also think the very concept of the friends list might be useful for people who are lonely or have low self-esteem. It can be a pick-me-up.