It is important to keep your voice hydrated especially when you are performing, the reason for this is because when we sing the protective lining which cats the vocal chords protects the friction that is caused on the voice when we sing. If you do not drink enough water when you have been performing and during your performance then the vocal chords may begin to swell and then they do not vibrate as freely as they should which then has an affect on your performance. You can cause permanent damage to the voice if it is not hydrated enough and still put under pressure.
It normally takes about twenty minutes for water to be properly absorbed in your system as it goes to things like your heart and lungs first, so your vocal chords are on of the things that does not get hydrated till last therefore it is important to be drinking water twenty minutes before your performance.
If your vocal chords do swell after you have had a big performance don't panic! just keep taking fluids
and this will flush out any lactic acid off the vocal chords and then they should return back to normal.