stupid question but I need to know what this part in the picture is called. figure in pic is on it. it sits behind the rotor where the back breaks are. and if its really needed because ive been told it is needed and it isn't needed

 

sorry finger is on it

 

Backplate.

 

Some think like dust cover, backplate. You need it cos brake shoes line up against it and secured to it by retaining clips. They cost as far as I've found out over £100 each.
Right 5073 640 AA.
Left 5073 641 AA.
Secondhand ones likely to be very rusty as well.