to pull vapors from the canister back into the tank. Tank creates a vacuum as the fuel is pushed out and into the engine. If return system is clogged - that can't happen.
FYI -- I once had another car where the vapor canister became clogged and every so often the car would stall until I opened the filler cap to relieve the pressure. It was a non fuel injected system, so not as much drama with the tank...