What am I missing?

I'm trying to work this out.

I suppose it saves pinning the whole piece or pieces of material together.
It keeps the seam straight as it guides the fabric along the card
It keeps the fabric in place better.

I won't know unless I try it. But not today 😀

I’m giving it a go this afternoon ( hopefully)
And strangle enough I’ve just had a lamp delivered where a piece of the packaging was that piece of cardboard.

The only thing I can suggest is that it's giving you an even hem allowance but I still don't understand why you should weave the fabric through the pins

I suspect the pins are a mild tensionning device and the cardboard is a good edge to guide aagainst.

I haven't tried this, as my machine would bend the pins for certain!

I assume the advantage of using the card is that the cloth stays in line with it, easier than if you just follow the seam guide.

Weaving the elastic over and under the pins presumably keeps it taut, so you don't have to stretch the elastic with the hand that is not guiding the material.

I gave it a go but couldn’t see any advantage. I use a magnet on the rare times I need an extra wide seam.

nanasam

The only thing I can suggest is that it's giving you an even hem allowance but I still don't understand why you should weave the fabric through the pins

It may be to stop the fabric slipping.