The creepage distance is important when electric potential reaches high level. Also a single electric discharge could irreparably destroy or damage the linked components.
The electrical discharge between two traces or vias on a PCB is superficial, if it strikes once it can leave a conductive residual on the path, reducing even more the trigger voltage.
Protection of the traces
Minimal distance between traces (creepage distance)
The creepage distance given is the minimal and it is valid with normal values of humidity level and clean air.
For particular ambiance, with high humidity level, is necessary to increase both the distance between traces and the thickness of the protective paint.
Hard working condition.
For PCBs destined to work in industrial ambiance, they must be always protected with coating paint. Dusts and conductive fumes cause frequent damages and lacks
Starting by 200-300 V, it becomes important the dirt deposited on the critical point during the working phase. If the equipment must work for years without any cleaning operations, it is advisable to add a dirty factor if 1.5 or 2.