Why break a PCB trace plane into multiple parallel traces?

A solder proof mask is usually used to cover unconnected areas of copper in a circuit to prevent solder from tinning the copper during the manufacturing process. This type of circuit is soldered by running the preheated pcb over a wave of solder so that the whole board back is exposed to solder.

In circuits where the copper has to carry large amounts of current the copper connections are left exposed so that they get tinned. This increases the metal thickness and hence the current carrying capability.

Tinning a large single area of copper is difficult and will lead to an uneven thickness of solder over the copper area. By breaking down the area into individual unmasked "tracks" the tinning process works much better.