Backing plate seal and stud tool question.

The foam is porous. It compresses to much thinner and more dense where bolted and fill voids between the mating parts everywhere else. Moisture that does get in can evaporate out. Thin sheet metal would probably rust away faster without the foam. After 50+ years, of course there will be some rust. I suppose if you painted all these parts that weren't painted the foam wouldn't be needed.