It's mainly for pliability. If it's bone dry the vibrations are more "brittle" and the reed doesn't doesn't marry well with the aperture of the mouthpiece. I only soak mine for a minute or so and usually with saliva, though some drench them by soaking them in a glass of water. When I have not used the sax for a few songs and it's time to play it I will work up as much saliva as possible and hold it in my mouth to soak it while waiting for the time to play. They don't dry out THAT fast where a song or two will matter that much.