Why we need the PCB Panelised for assembly?
The need for a larger number of boards per panel may simply be a requirement of the pick and place machine, which requires a particular panel size, and two PCB's may be wasteful of space.

As Lior mentioned in a comment, V-scoring can be problematic depending on the board thickness.  It is also not particularly recommended for boards with surface mount components, as the bending of the board to break it off at the scoring line can put stress on the components.  It is possible to get around this by sawing along the scoring line, but this takes a lot more time.

Instead, we have separated the boards from the rest of the panel, and the boards from themselves, by having a router mill out a space between them; and the boards and panels are then connected together by "mouse bites", which are much easier to separate than a scored line running down the entire length of the board.



A second requirement for most pick and place machines is to have a set of fiducial markers located on each board.  Usually they are placed in three corners of the board, like this:



By using thee, rather than four, it is easy to tell if the board is oriented upside down, and three are all that are necessary to fix the location of the board in both X and Y directions.

As shown in the photo, each fiducial is usually just a circle of bare copper, inside a slightly larger circle of overlapping solder mask.  A camera on the pick and place machine accurately locates each fiducial and uses this information to accurately place all of the components.