Why 0R used in voltage lines in series?
Why these resistors used as series to the power lines? Need them
  really?


If you have multiple circuits teeing off the same power rail and the overall circuit is quite complex, there is a good reason for putting zero ohm links in series with the individual power feeds and that reason becomes apparent when fault finding. 

If one circuit is dragging the whole power rail down (due to a shorted sub-circuit somewhere), having zero ohm links helps to track down the fault because they are easily removable, easily replaceable and cheap.

Personally if the individual load current requirements are moderate and you could bear the cost of a 0.1 ohm resistor (instead of zero ohms) you could probably identify the rogue sub-circuit using a DVM across each zero ohm link. Bear also in mind that zero ohm links might be sufficiently "resistive" to facilitate this nicety.

The Vishay CRCW zero ohm link has a maximum resistance of 0.02 ohms so I suspect it could be somewhat relied upon to be about 0.01 ohms. 2 amp through 0.01 ohms is a volt drop of 20 mV and very easily seen on a modern DVM.