Noob question: What's the foam inside the sub's do?

[Prismatic]

Not foam, but kinda like the stuff you fill a pillow with? I just took an old Creative 2400 sub box, and added another sub to the box (it's a cube, so I added another sub on another side). But was wondering if I should put that packing foam stuff inside? Hard to describe it 🙁

 

[454Casull]

It increases the box volume (not physically, although the box appears larger to the driver), if used in a sealed box.