I think its just for sound deadening.
I keep cheking mine over winter and dry it out when nessersary. I cut it all out to save having to take the carpet out each time.
Now summer is here and its warm inside the cabin I aint so worried as they dry out naturally.
I have also tried to find the leak without success, although I think sealing the windscreen surround helped.
Thanks - was thinking of just leaving the whole lot out. Its an off road\green lane car anyway so a bit more noise is not an issue. Do you know if there is anything I can put down instead of the carpet and sponge?