Why is a step-down transformer used instead of a resistor for reducing high voltage? For example: If input voltage is 230V and I have step down the voltage, can I use a resistor instead of transformer? If I cant, can any one explain the drawbacks?
Think of a transformer like a speed reduction gear box - high speed, low torque in and low speed, high torque out.
Think of a resistor like a constantly slipping clutch - it burns power to reduce speed.
Ideal resistors dissipate power as heat to drop voltage.
Ideal transformers don't have power losses to transfer to a lower voltage (real transformers do due to winding resistance and core losses, but these are typically significantly less than resistors).
Additionally, the voltage drop of a resistor is dependent on the load impedance/current draw while transformers are generally immune to this.
How much a resistor drops voltage is dependent on how much current is flowing through it (Ohm's law). As loads are often variable, this means that the voltage drop will be variable as well. There are a number of applications where a variable supply voltage is unacceptable for various reasons.
If you are looking to build a transformer-less power supply then you should look at capacitive drop supplies, where the charging curve of a capacitor is used in conjunction with a zener diode to restrict voltage to a maximum level. But keep in mind that capacitive drop supplies lack the isolation that a transformer provides, and as such are highly dangerous for inexperienced electronics students and they should not attempt to build one.