Yes, even your best friend from the bar counts.

Why do legacies do this? Well, because it's pretty annoying to have to stand in two seperate lines when it's completely unnecessary. Sure, it's not the end of the world but it's one of those things that's not hard to do so you might as well do it.

When I travel, anyone who I know on the flight gets:

Elite Check-in
Elite Security Line
Priority Boarding
Priority Seating

even if they are not on the same PNR. As long as I tell the agent I'm a DL gold. They'll assign them exit row because they are in my party.