why is the bulb glows suddenly when the switch is on,though the electrons move at adrift speed?
Why is the water stored so high up? It took work to lift it up there. That work is stored as potential energy. When the water is allowed to flow down a pipe and out, it is moving fast because the potential energy has been converted to kinetic energy. Notice what matters here is the difference of the height at which it was stored and the height at which it is used.
Similarly, when charges have been separated, work was done. The stored energy per charge is the electric potential difference or sometimes just called the potential.
Consider a whole field of water balloons. The ones thrown the highest will be moving the fastest when they come back down.Now consider two water tanks. Water from the tank that was raised higher will be moving faster when it exits the pipe. Similarly, if we put more work into separating charges, we will give them greater potential. The charges with higher potential will be moving faster when we let them go in the completed circuit, if everything else is held to be the same.