First of all, not such an obvious question, if you know anything about water heaters, other than the fact that there are gas and electric models, you would know that the only type of water heater that has a switch is an electric unit. Hilda said the water heater is in a cupboard, which means it's fairly small, probably less than 20 gals. These units are used in mobile homes because they can be located under kitchen cabinets and out of the way. They are also used in some new home construction for instant availability of hot water, in this case they are called "point of use" units. So, to the actual question. If you use a lot of hot water during the day, for what ever use, then leaving the unit on for that period is practical. You have the hot water available when you need or just want it. If demands for the hot water taper off during the evening hours, maybe then you could shut it down for several hours without any inconvenience. You would have to remember to turn it back on upon rising the next morning, so that the hot water is again available.
40 + years of home building, remodeling, repairing, and maint.