This is a tool to get on/over thicker seams and do other things like helping you sewing on buttons. (See this manual on page 4.17. )
Other links :
Number 1
Number 2
Since my wife told me this a too expensive piece of plastic, I made my own one.
To be printed by preference in ABS and treated with aceton vapor to get a smooth surface.
Picture shows the printed version next to the original grey Pfaff one. This one is printed in PETG and sanded to smooth the surface.