Should I fill this gap between floor boards and wall?

Currently fitting some new skirting board, I have the opportunity to fill some gaps that exist between the original floorboards and the brick wall. (1930s build). I’m considering squirting some expanding foam in this gap (mainly for insulation, but also mice) before fitting the new skirting.

My question is whether I should. Is there’s a good reason to leave this gap and not fill it in? Possibly the gap is there for a purpose. Ventilation, damp etc?

Not easy to see in the image, but may help to describe. That’s a modern laminate floor on top of the old boards. The large gap is one of several all around the room where a brick has been left out probably deliberately. It’s not this I am considering filling, but the thin (1 inch or so) gap indicated by the yellow arrows.

Below the floor boards is a void of about half a metre until you hit soil, which seems to be dry. There’s no cellar and I have no access down there without pulling up the floor. None of the walls are external, except one used to be before an extension was added.

Fill it or leave it?