What is the usage of Zero Ohm & MiliOhm Resistor?
There's a huge difference between a 0 &Omega; resistor and a 1 &Omega; resistor: the latter has an infinitely larger resistance :-).

The 0 &Omega; has different usages:


selective connections. You can create variants of your circuit by placing or leaving out the jumper. Just like you would delete a connection in your schematic capture program (= remove jumper) and make a connection to a different point (= place jumper)    
facilitate routing. A couple of jumpers over traces may allow you to use a single layer board instead of a double layer, which would cost you more. You'll typically use 0603 or 0805 size jumpers for this; 0402 are too small to bridge an average trace.
provide a current measurement point. During development and testing you can place a low resistance shunt resistor to measure the current, and for production replace it with a zero ohm jumper. Then you don't have to cut traces to insert the shunt resistor in the circuit. Probably less applicable, since you should have measured the current before creating the final PCB, but for very low current circuits the layout and PCB material may matter, and then you do want to measure on the final board.