plastic piece

What window related part is this? Pulled the drivers side panel off to grease the window rolling mechanism. Found this at the bottom of the interior of the door. What is it? How important?

Hi guys, I broke the plastic clip of death for the tailgate... (see photo) Turns out after investigation the control arm thing isn't connected anymore on the passenger side. I think the plastic clip broke, is there a way to replace this clip (or the entire rear passenger hinge if necessary??)...