M_Boats: Bilge Foam

My M15 had a pile of expandable foam up against the partial bulkhead about a foot behind the centerboard housing. The PO didn't know why it was there and pointed the finger to the original owner. Yesterday I removed the foam and found that there is a 4" square well just forward of this partial bulkhead. My questions are; What is the purpose of this indentation and why would someone fill it with foam? I've not encountered any water there or anywhere in the cabin. Thanks, Rick Langer M15 #337