Central Heating combi boiler. immersion switch still require?? or is it just the new on/off switch?

I have just had an old back boiler removed and a completely new Central Heating system put in with 7 thermostat controlled radiators and a combi boiler for instant hot water etc, however im confused as to why the gas man didn't remove the the old immersion heater switch as this i have been told needs to be left on...as if i turn it off the boiler goes off? is this true? why would i need this on for a combi boiler??