Why do large PCB traces have solder points/islands/stripes on them?
Probably an attempt to reduce the resistance of the trace (and better dissipate heat) by putting solder in parallel. 

The pattern in the solder mask is likely so that a solid section of solder does not warp the board excessively as it cools (the CTE of solder is much higher than that of FR4). 

You can see shorter traces to the right that are covered almost entirely by solder.