In many PCB board, I have seen zero ohm resistor in between the signal lines. What is the reason behind it? Can't we simply connect it through copper trace?
The zero ohm resistor is simply a jumper that can be conveniently populated with pick and place machinery. Sometimes the board is too tight to route without crossing sides a zillion times, and a simple jumper can really make the routing much nicer. Other times, you might want very tight control, making sure that two ground planes, for example, touch at only one point. A jumper can make this simple and apparent. The resistor is simply a jumper that can be machine handled.
Several possible reasons:
In 1 or 2 layer PCB designs, sometimes 0 ohm resistors are used to jump a signal across trace routing bottlenecks.
0 ohm resistors are used "just in case". Just in case the designer needs to do something with the design later. As in, "I probably don't need this but if something doesn't go as expected I might need to change this to a 1K ohm resistor".
0 ohm resistors are often used as "soldered in jumpers". Board configuration options that are set when the PCB is manufactured, not by the end user. Sometimes, one board will be used for several slightly different products. 0 ohm resistors can configure the board.
We add zero-ohm resistors to all of our supply rails. Recently we had a board with TEN different voltage rails on it: -6v, 1.8v, 2.5v, 3.3v, 3.6v, 4.1v, 5.0V, 9.6v, 12v and 18v. Some are bias voltages, and only require a few hundred microamperes.
When the first article comes back from the manufacturer, we remove all of the zero-ohm resistors and bring up each rail, one at a time. When we are satisfied all the voltages are correct, we then power the circuits up, wiring the pads of the zero-ohm resistor through a milliammeter, and verify nothing is shorted. (Unfortunately some times you have to add in several rails at the same time, e.g. a TFT LCD which takes six different voltages)
Zero ohm resistors are used where the designer my want to modify the circuit after it is built. It is easier to desolder a component than it is to cut traces. They may also be used inplace of vias and double sided PC boards. A zero ohm resistor can jump over a number of other traces where a trace on the board would have to route around.
Sometimes, a circuit isn't possible on a one-layer PCB, so to reduce costs, they insert a wire bridge. It could also be that the PCB design is otherwise so complex that they would have to use thinner traces, which they don't want.
Most of the time, they don't use wires but zero ohm resistors because then the insert-resistor machine can place them on the PCB.
It might also be that the designer wants to leave the opportunity open to use a resistor of a significant value later on.