What type of switch should be used for water heater? I read that it requires a double pole switch. Why is it so?

Cant we just use the normal switch, like switch for lightings? is it dangerous to use this?

Lighting circuit accessories is usually rated at ard 5Amps (5A). Water heater is usually 3KW, or 13A. Water Heater switch is rated at 20A.

If u use normal power point to operate water heater, can see signs of arcing (like lightning) when switch is activated when the water heater is running. This will substantially reduce the life cycle of the switch. Purpose built water heater switch no such issue.

Put it this way.. cost is not much difference in absolute $ terms. Downtime of a water heater is quite disruptive to family life.