because on a metered system like an ERST it don't like leaks as you have air passing the metering head but not actually going into the engine, hence why the air returns to the system past the metering unit.

on a cossie it's map based, so it doesnt matter if any air goes amiss...

same reason you cant run "normal" dumpvalves on ERSTs ST Esccos etc. they just dont like leaks

altho its such a tiny leak it really makes fack all difference... lol i ran my old TT (metered system) without bleeding the air back in quite happily... lol but then that car was held together with gaffer tape and cableties