If you need to do repair work on your breaker box, electrical wiring or you want to install a sub panel, it is very important to turn off the power to the home before attempting the work. It's easy to assume that once the mains are turned off that you are now completely safe to start working, but experienced electricians know a volt meter should be used to test the electrical panel to make sure it is free of electricity.
Even if your volt meter test shows that no power is going to your circuit breakers, there is still a danger of electrocution. The main power cables leading into your circuit box remain energized even though the mains are in the off position. The big, heavy black cables themselves should never be touched under any circumstances. They are live, but simply are not feeding power to the rest of the circuit. When your mains are in the off position they keep the electricity from flowing through the circuit, but the voltage is still there in the cables, waiting to flow again.