Some questions about the RECOM RAC02-05SC AC/DC power supply converter
You haven't provided a schematic, so we don't know what is connected to those traces.  However, this certainly has the look of increasing the creepage distance to minimize leakage across some isolation barrier.

Circuit boards are made from insulating material.  But, stuff happens, and after some use, dust and dirt can get on the PCB.  Add some humidity in the air, and you can have resistance orders of magnitude lower between two points than would be if the board were clean.

In some applications, like patient-touching medical devices, a few &micro;A of leakage you can usually ignore become significant.  One way to reduce the leakage current despite inevitable dirt and moisture is by increasing the creepage distance.  That's the shortest distance along the surface of any object between two conductors.  Cutting a slot in a PCB can greatly increase the creepage distance between two critical nets.

Cutting slots in a PCB is a extra step in manufacturing, but is usually cheaper and more desirable than making the device larger.