The quick-release valve is a little lever on the top of a storage water heater.  It is for safely releasing excessive temperature and pressure inside the storage tank. Rheems's water heaters have a built-in system to automatically monitor and control the temperature and pressure. However, for added safety, they also come with a relief valve.
Consequently, without the quick-release valve, the water heater may run the risk of an explosion. Most especially if the temperature or pressure got higher than what the tank was designed to handle.
To put it simply, storage water heaters can be dangerous or even lethal if they do not come with a temperature and pressure relief valve.