Power supply lines in a chip are designed to be very wide so that EM and resistance effects are minimized. However, there is one problem with very wide metal lines that occurs when the temperature of the chip rised high enough to cause the metal to expand signigicantly.
If the metal expands repeatedly with enough force, the metal
will destroy the isolation and passivation layer that protect the wafer. Impurities and particles will work their way onto the chip, react with the different materials, and cause the chip to fail or work unreliably.
To address this problem, layout designers are required to put slots or holes in the metal at regular intervals.
The origin of this rule is found by the assembly due to high temperature in assembly.