I have noticed this strange knob on flights typically near the video screen (in this case on the right and is underneath the USB port).
Is there a use to this knob or is it just a component which has remained in airplane design for some reason or the other and is not required for use by the passenger?
Typically it just comes out to a certain length and goes back in, does it do something else as well?
It is a hanger, you can hang your jacket or anything similar there. The same exact ones typically available in lavatories for passengers and in galleys for crew members. In the passengers cabin, they are usually available in first or business classes' seats and it comes with a sign:
I guess they forgot to add the sign, making it harder for passenger to know how useful it is.
Why is it retractable?
To make it hold tight to your jacket or whatever you hanged on there, so it will not fall easily with airplane movement or when you get in/out of your seat.
