Whale water heater isolation switch

Above the near side sliding door on my AS Kemerton is a panel with an on/off switch which says: ‘fresh water tank isolation switch’. Can anyone explain, when and for what purpose would I need to use this isolation switch?
Mel

Before anyone replies, I have also found a waste water tank isolation switch under the seat at the offside of the cab which I guess [from checking on other threads] is a 'frostat' switch to prevent freezing of water in the tank.
Mel