The charcoal canister "captures" hydrocarbons from fuel tank vapors. When the engine is running, the captured vapors are drawn out of the canister and into the engine so they can be combusted and run through the catalyst.
The PPS basically regulates the flow of purge vapor from the charcoal canister to the engine. I'd guess the valve regulates flow in a propotional manner to a control current or voltage commanded by the ECU.
It's all emissions, trying to keep everything green.