The service manual could tell you everything about what is going on.

In simple terms:

The HiHy keeps the fuel system pressurized at all times. Gasoline under pressure doesn't evaporate. This helps the car achieve a PZEV rating. Partial Zero Emissions Vehicle cannot have any evaporative emissions.

The old HiHY and the Camry Hybrid do the same thing. The Prius uses a bladder to keep excess air out of the tank.