The "friend" feature has been in effect since the rollout of the "old new" Nest/Knot. This was when the site introduced blogging, etc.
It's sort of MySpace/Facebook-like. It doesn't serve much of a purpose, in my opinion. I basically accept anybody who wants to be my "friend." On the "old new" Nest/Knot, you could only send private messages to "friends."
And, for those of you who weren't already aware, I'm an annoying know-it-all because I was one of many beta-testers for The Knot/Nest before the rollout of the blogging feature.