In buck circuit, If I mount 0E resistor instead of inductor then what will be the problem ?
And what is the use of inductor in buck regulator ? whether just for filtering ?
You can think of a switching regulator (such as a buck converter) as a digital pulse generator followed by a low-pass filter which averages out the pulses to create a DC voltage. The normal inductor-capacitor network that you see in a buck converter is simply a second-order low pass filter and indeed you could replace it with a first-order resistor-capacitor filter if you wanted.
However the quite obvious problem with doing this is that it would be extremely inefficient because the resistor creates an impedance by dissipating energy whereas an inductor creates an impedance by storing energy. Indeed you will have essentially just made a chopper style linear regulator, which doesn't offer any advantages I can think of over using a regular linear regulator while being much more complex.
This is why you don't see such a design for power supplies in practice.
The inductor is required. It is a key component.
During ON time current is coming from source. During OFF time current is coming from the action of the inductor.
When ON turns OFF, current in inductor stops. No current, so magnetic field will collapse. Faraday's Law states a voltage will be induced across inductor (- +), which forward biases the diode and current flows through the load.
Now before you say, we are getting something for nothing in the OFF state, you must realize the inductor was charged during the ON state.
What we are trying to achieve is current in the inductor never reaches 0. No inductor and the circuit will NOT work.
You could, theoretically, use an RC filter instead of an LC filter on the output of a buck converter. As Jon mentioned, this is very inefficient. But in order for it to work there has to be some resistance. A zero-ohm resistor won't work. If you try to use a zero-ohm resistor, the capacitor will short out the pulse voltage during transitions. That may be what causes your regulator to fail over time.