Because you can never be too prepared…
The vacuum sealed bag is not needed. A stock Glock will work perfectly well when wet!
If you are planning on being in very wet environments often though it is recommended to swap your regular spring cups (recommended only for use on 9mm Glock models) for special “maritime spring cups”. This allows water to more freely pass by the firing pin within the firing pin channel. Having regular spring cups in might hinder the drainage of water and create a hydraulic force within the firing pin channel which in turn would slow the firing pin down enough to cause an FTF (Fail To Fire) because of a light primer strike.
Spring cup location (you’re looking at the firing pin and firing pin spring with one spring cup removed):
Regular spring cups:
Maritime spring cups (notice the channels it creates, where water can drain):