I suspect the plastic "firewall" pass-thru is for convenience of
manufacturing. Without it they'd have had to leave (at engine install
time) a long dangling fuel line which either needed to be cut to fit
whatever type engine they were putting in if attached to the body, or
have someone crawl underneath and string and fasten fuel line if
attached to the engine. The dangerous plastic fitting allowed them to
have a short bit of hose on the engine and plug it right there. For
us, with only our one Vanagon to deal with, it's just a liability. I
replaced mine with a plastic sleeve stuck(like on the return side)
through a rubber grommet, and a continuous hose going through it.

> Turned out the leak was coming from the back side of the plastic barb
> fitting that takes fuel thru the firewall into the engine
> compartment. 
> Anybody know what the heck this barb fitting is supposed to be good
> for?  Other than adding two potential leak sites to the fuel system!  I
> think it is like the human appendix... a useless relic of evolution,
> usually does no harm but if anything were to go wrong you really wish you'd
> had it OUT already.  If replacing fuel lines I would say get rid of this
> fitting and run a single piece of fuel line from the pump all the way back
> to the engine.