The EVAP canister vent valve is part of an emission control system that traps vapors coming from the fuel tank and allows them to be burned inside the engine rather than be released into the environment. 

Gasoline that is allowed to evaporate into the atmosphere is not only wasted but adds emissions that can be bad for the environment and harmful to humans. The EVAP system allows the fuel tank to be vented so that pressure does not build up inside, while at the same time preventing the gasoline vapors from escaping.

The purpose of the vent valve in this system is to close when the engine control module (ECM) performs leak testing on the EVAP system, thereby creating an airtight apparatus. When this happens, the ECM monitors the rate of drop in the pressure of the system, which determines if a leak is present and the severity of the leak. Without the canister vent valve, this test would not be possible.