My best guess would be familiarity, a connection to the real world. When you go into a physical location, say visiting a hospital or business, you "sign in" an

My best guess would be familiarity, a connection to the real world. When you go into a physical location, say visiting a hospital or business, you "sign in" and "sign out" of that place. When you participate in an activity, for example a class or sports club, you "sign up" to join.