I'm with zed on this, I've got a rotten '71 panelvan and I took the firewall panel off recently and was obviously the first to do it. The seal if undisturbed is very effective and the tank area was spotless, like a new van when the rest of the engine bay was filthy.
Poundland self adhesive draft excluder seal works for me, I also think its just to stop vibration, and possibly add an albeit small bit of protection, as a fire delay in the event of an engine fire or rear end shunt, its just a flimsy closing panel partition at the end of the day.
