"Is this necessary?"
No but might be wise. There is remote possibility reducing heat seting might alow a poorly insulated pipe to freeze,burst and flood the house. Washing machine supply hose have a nasty habit of rupturing when noone is home. In addition to turning main off water heaters should be set to "vacation". I don't turn water heaters off because that somtimes causes drain to drip when it returns to operation. Both methods are to prevent boiling heater dry.

Turning off the water is pretty cheap insurance. I can't think of a reason not to do it.