Thanks for your excellent question! I wondered that also, but never thought of a good reason to have one.
I was able to check our product guide on the SU Demonstrator Website, and here is what it stated:
Foam Brayer Attachment
This soft foam brayer attachment snaps into a brayer handle. Use it to create stenciled shapes and backgrounds by rolling it on an ink pad, then rolling it over the Stripes Stencil, or any other stencils you choose. Use it to create a lovely, soft sponged look. For a watercolor effect with the foam attachment, ink brayer on a Spectrum pad, mist the roller with water, and apply to paper.
Hope that helps!